Jay Last, a physicist who helped create the silicon chips that power the world’s computers, and who was among the eight entrepreneurs whose company laid the technical, financial and cultural foundation for Silicon Valley, died on Nov. 11 in Los Angeles. He was 92.

His death, in a hospital, was confirmed by his wife and only immediate survivor, Debbie.

Dr. Last was finishing a Ph.D. in physics at the Massachusetts Institute of Technology in 1956 when he was approached by William Shockley, who would share a Nobel Prize that same year for the invention of the transistor, the tiny electrical device that became the essential building block for the world’s computer chips. Dr. Shockley invited him to join a new effort to commercialize a silicon transistor at a lab near Palo Alto, Calif., about 30 miles south of San Francisco.

Dr. Last was awed by Dr. Shockley’s intelligence and reputation, but unsure about the job offer. Ultimately, he agreed to join the Shockley Semiconductor Laboratory because it sat in the Northern California valley where he had spent a summer harvesting fruit after hitchhiking there from his home in Pennsylvania steel country.

But he and seven of his collaborators at the lab clashed with Dr. Shockley, who later became infamous for his theory that Black people were genetically inferior in intelligence to white people. They quickly left the lab to create their own transistor company. They later came to be called “the traitorous eight,” and their company, Fairchild Semiconductor, is now seen as ground zero for what became known as Silicon Valley.

At Fairchild, Dr. Last led a team of scientists who developed a fundamental technique that is still used to manufacture computer chips, providing the digital brains for billions upon billions of computers, tablets, smartphones and smartwatches.

“There was nothing more important than Fairchild Semiconductor to the Silicon Valley experience as we know it today,” said David C. Brock, a curator and director of the Software History Center at the Computer History Museum in Mountain View, Calif. “Many of the dynamics that still persist were crystallized by the founders of Fairchild, and Jay was right in the middle of it.”

Jay Taylor Last was born on Oct. 18, 1929, in Butler, Pa. His father, Frank, a German immigrant, and his Scotch-Irish mother, Sarah, had met when they were two of the three teachers at a high school in Ohio. After they married, Frank Last felt he could not support a family on a teacher’s salary, so they moved to Pennsylvania, where he went to work in the new Butler steel mill, not far from Pittsburgh.

Jay Last grew up in Butler before making his first pilgrimage to the West Coast when he was 16. With the blessing of his parents — and carrying a letter from the local police chief saying he was not running away from home — he hitchhiked to San Jose, Calif., which was then a small farming town. He had planned on making a little money picking fruit, but he arrived before the harvest began.

Until it did, he lived, as he often recalled in later years, on a nickel’s worth of carrots a day. Whenever he faced a difficult situation, he said in an interview for the Chemical Heritage Foundation (now the Science History Institute) in 2004, he told himself, “I got through that when I was 16, and this is not that bad a problem.”

At the suggestion of his father, he soon enrolled at the University of Rochester in New York State to study optics — the physics of light. During summers back home in Pennsylvania, he worked at a research lab that served local plate-glass manufacturers.

Fulfilling a promise he had made to himself as a teenager, he went on to get his doctorate at M.I.T., before returning to Northern California and joining the Shockley lab. But he chafed at Dr. Shockley’s overly attentive and controlling style of management.

“I was a laboratory assistant, and that’s the way he was working with everybody,” he remembered in 2004. “There was no such thing as everybody getting together in a seminar and discussing what we were doing.” After about a year, he and his colleagues left to form Fairchild Semiconductor.

Using materials like silicon and germanium, Dr. Shockley and two other scientists had shown how to build the tiny transistors that would one day be used to store and move information in the form of an electrical signal. The question was how to connect them together to form a larger machine.

After using chemical compounds to etch the transistors into a sheet of silicon, Dr. Last and his colleagues could have cut each one from the sheet and connected them with individual wires, much like any other electrical device. But this was enormously difficult, inefficient and expensive.

One of the founders of Fairchild, Robert Noyce, suggested an alternative method, and this was realized by a team Dr. Last oversaw. They developed a way of building both the transistors and the wires into the same sheet of silicon.

This method is still used to build silicon chips, whose transistors are now exponentially smaller than those manufactured in the 1960s, in accordance with Moore’s Law, the famous maxim laid down by another Fairchild founder, Gordon Moore.

With Dr. Last’s death, Dr. Moore is the last surviving member of the “traitorous eight.”

The leaders of Fairchild Semiconductor would go on to build several other chip companies, including Intel, co-founded by Dr. Moore, and Amelco, co-founded by Dr. Last. The company’s founders and employees would also create some of the leading Silicon Valley venture capital firms and personally invest, as Dr. Last did, in many of the companies that sprouted up in the region over the decades.

Dr. Last retired from the chip business in 1974 and spent the rest of his life as an investor, an art collector, a writer and an amateur mountain climber. His collection of African art was donated to the Fowler Museum at the University of California, Los Angeles, and his trove of California citrus-box labels — an echo of his teenage summer in Northern California — is now at the Huntington Library, Art Museum and Botanical Gardens in San Marino, Calif.

As Dr. Last was finishing his Ph.D. in 1956, he was asked to take over as head of the glass lab back in Butler, Pa., where he had worked during the summers. It seemed like a promising opportunity.

“I went and told my parents,” he remembered. “My mother said, ‘Jay, you can do a lot better than that with your life.’”

KISANFU, Democratic Republic of Congo — Just up a red dirt road, across an expanse of tall, dew-soaked weeds, bulldozers are hollowing out a yawning new canyon that is central to the world’s urgent race against global warming.

For more than a decade, the vast expanse of untouched land was controlled by an American company. Now a Chinese mining conglomerate has bought it, and is racing to retrieve its buried treasure: millions of tons of cobalt.

At 73, Kyahile Mangi has lived here long enough to predict the path ahead. Once the blasting starts, the walls of mud-brick homes will crack. Chemicals will seep into the river where women do laundry and dishes while worrying about hippo attacks. Soon a manager from the mine will announce that everyone needs to be relocated.

“We know our ground is rich,” said Mr. Mangi, a village chief who also knows residents will share little of the mine’s wealth.

This wooded stretch of southeast Democratic Republic of Congo, called Kisanfu, holds one of the largest and purest untapped reserves of cobalt in the world.

The gray metal, typically extracted from copper deposits, has historically been of secondary interest to miners. But demand is set to explode worldwide because it is used in electric-car batteries, helping them run longer without a charge.

Outsiders discovering — and exploiting — the natural resources of this impoverished Central African country are following a tired colonial-era pattern. The United States turned to Congo for uranium to help build the bombs dropped on Hiroshima and Nagasaki and then spent decades, and billions of dollars, seeking to protect its mining interests here.

Now, with more than two-thirds of the world’s cobalt production coming from Congo, the country is once again taking center stage as major automakers commit to battling climate change by transitioning from gasoline-burning vehicles to battery-powered ones. The new automobiles rely on a host of minerals and metals often not abundant in the United States or the oil-rich Middle East, which sustained the last energy era.

But the quest for Congo’s cobalt has demonstrated how the clean energy revolution, meant to save the planet from perilously warming temperatures in an age of enlightened self-interest, is caught in a familiar cycle of exploitation, greed and gamesmanship that often puts narrow national aspirations above all else, an investigation by The New York Times found.

The Times dispatched reporters across three continents drawn into the competition for cobalt, a relatively obscure raw material that along with lithium, nickel and graphite has gained exceptional value in a world trying to set fossil fuels aside.

More than 100 interviews and thousands of pages of documents show that the race for cobalt has set off a power struggle in Congo, a storehouse of these increasingly prized resources, and lured foreigners intent on dominating the next epoch in global energy.

In particular, a rivalry between China and the United States could have far-reaching implications for the shared goal of safeguarding the earth. At least here in Congo, China is so far winning that contest, with both the Obama and Trump administrations having stood idly by as a company backed by the Chinese government bought two of the country’s largest cobalt deposits over the past five years.

As the significance of those purchases becomes clearer, China and the United States have entered a new “Great Game” of sorts. This past week, during a visit promoting electric vehicles at a General Motors factory in Detroit, President Biden acknowledged the United States had lost some ground. “We risked losing our edge as a nation, and China and the rest of the world are catching up,” he said. “Well, we’re about to turn that around in a big, big way.”

China Molybdenum, the new owner of the Kisanfu site since late last year, bought it from Freeport-McMoRan, an American mining giant with a checkered history that five years ago was one of the largest producers of cobalt in Congo — and now has left the country entirely.

In June, just six months after the sale, the Biden administration warned that China might use its growing dominance of cobalt to disrupt the American push toward electric vehicles by squeezing out U.S. manufacturers. In response, the United States is pressing for access to cobalt supplies from allies, including Australia and Canada, according to a national security official with knowledge of the matter.

American automakers like Ford, General Motors and Tesla buy cobalt battery components from suppliers that depend in part on Chinese-owned mines in Congo. A Tesla longer-range vehicle requires about 10 pounds of cobalt, more than 400 times the amount in a cellphone.

Already, tensions over minerals and metals are rattling the electric vehicle market.

Deadly rioting in July near a port in South Africa, where much of Congo’s cobalt is exported to China and elsewhere, caused a global jump in the metal’s prices, a surge that only worsened through the rest of the year.

Last month, the mining industry’s leading forecaster said the rising cost of raw materials was likely to drive up battery costs for the first time in years, threatening to disrupt automakers’ plans to attract customers with competitively priced electric cars.

Jim Farley, Ford’s chief executive, said the mineral supply crunch needed to be confronted.

“We have to solve these things,” he said at an event in September, “and we don’t have much time.”

Automakers like Ford are spending billions of dollars to build their own battery plants in the United States, and are rushing to curb the need for newly mined cobalt by developing lithium iron phosphate substitutes or turning to recycling. As a result, a Ford spokeswoman said, “we do not see cobalt as a constraining issue.”

Increased mining and refining of cobalt by Chinese companies has helped meet the growing demand and advanced the fight against climate change. But as more electric vehicles are produced by more automakers worldwide, the International Energy Agency expects a cobalt shortage by 2030, based on an analysis of existing mines and those under construction. Other forecasters say a shortage could hit as soon as 2025.

A review by The Times of documents filed with regulatory authorities in China shows the acquisitions in Congo have followed a disciplined playbook, announced with great fanfare by Beijing in 2015, to dominate the world’s emerging clean energy economy.

As of last year, 15 of the 19 cobalt-producing mines in Congo were owned or financed by Chinese companies, according to a data analysis by The Times and Benchmark Mineral Intelligence. The biggest alternative to Chinese operators is Glencore, a Switzerland-based company that runs two of the largest cobalt mines there.

These Chinese companies have received at least $12 billion in loans and other financing from state-backed institutions, and are likely to have drawn billions more. In fact, the five biggest Chinese mining companies in Congo had lines of credit from state-backed banks that totaled $124 billion, according to the documents reviewed by The Times, even though one of them, China Molybdenum, described itself as “a pure business entity” traded on two stock exchanges.

China’s goal is to control the global supply chain from the metals in the ground to the batteries themselves, no matter where the vehicles are made. The approach, in part, echoes Henry Ford’s investments in Amazonian rubber plantations as the auto industry turned to mass production in the early 20th century.

The forested mine site at Kisanfu was just one of two major purchases in recent years by China Molybdenum. The first came in 2016, when it took control of Tenke Fungurume, a mine that on its own produces twice as much cobalt as any other country in the world. At least $1.59 billion of the $2.65 billion Tenke Fungurume price tag, financial records show, came from loans provided by Chinese state-owned banks.

As the Chinese were stepping up their focus on green energy in 2016, the soon-to-be U.S. president, Donald J. Trump, was extolling the fossil fuel industry, campaigning in West Virginia with a hard hat and shovel and falsely promising coal miners that “you’re going to be working your asses off!” After taking office, Mr. Trump would roll back requirements on American automakers intended to accelerate the transition to electric vehicles, giving the Chinese an even wider lane.

“It is pretty heartbreaking what happened here,” said Nicole Widdersheim, who worked on Africa issues for the National Security Council during the Trump administration. “Just so stupid.”

The frenzy for Congo’s cobalt has attracted an international cast of opportunists, luminaries and shadowy characters eager to benefit. At one point, it also drew in a Chinese-based private equity firm that Hunter Biden helped found and that was later scrutinized in the 2020 presidential campaign.

At the same time, Chinese companies are running into new headwinds from Congo’s government, according to documents obtained by The Times and interviews with current and former senior U.S. officials.

Congolese officials are carrying out a broad review of past mining contracts, work they are doing with financial help from the American government as part of its broader anti-corruption effort. They are examining whether companies are fulfilling their contractual obligations, including a 2008 commitment from China to deliver billions of dollars’ worth of new roads, bridges, power plants and other infrastructure.

Congo’s president, Felix Tshisekedi, in August named a commission to investigate allegations that China Molybdenum, the company that bought the two Freeport-McMoRan properties, might have cheated the Congolese government out of billions of dollars in royalty payments. The company risks being expelled from Congo.

At the Tenke Fungurume mine, there have long been problems associated with trespassers from nearby villages scavenging for cobalt. After China Molybdenum called on the government to help, Congolese troops fired on a trespasser inside the mine’s gates, killing him, as well as a second person who was shot after riots broke out in protest, witnesses and local officials told The Times.

Separately, at least a dozen employees or contractors at the mine told The Times that Chinese ownership had led to a drastic decline in safety and an increase in injuries, many of which were not reported to management. Two Congolese safety officers said workers were assaulted after they raised concerns and were offered bribes to cover up accidents.

“Things are falling apart in terms of safety,” said Alfred Kiloko Makeba, who retired last year after a decade working as a safety supervisor at the mine.

Vincent Zhou, a spokesman for China Molybdenum, rejected claims that the company had cheated the Congolese government or relaxed safety standards, saying the opposite was true, and questioned if there was an organized effort to undermine the company.

China has an idiom that goes something like: “Where there is a will to condemn, evidence will follow,” Mr. Zhou said in a written response to The Times. “Vaguely I feel that we may be caught in the gaming of greater powers.”

A Presidential Connection

African countries for years have been turning to China for help building infrastructure with loans or trades involving their natural resources — deals that analysts warn provide far more benefit to the Chinese.

A blueprint for those deals, now common across the continent, was sketched out in 2005 when Joseph Kabila walked into the Great Hall of the People in Beijing.

Mr. Kabila, then just 33, was the new president of Congo after the assassination of his father, another tragic milepost on the poverty-stricken country’s road of violence and political disruption.

China was familiar territory for Mr. Kabila, who had received military training there in the late 1990s. This visit was about enlisting the help of President Hu Jintao in turning around Congo’s economy.

The United States, which had long provided economic and military assistance to Congo, was locked in wars in Afghanistan and Iraq and had become increasingly uninterested in the country. Congo’s poor record on graft and human rights was also scaring away many international banks and Western investors.

Mr. Kabila’s wish list was long: He wanted new roads, schools and hospitals as part of a revival plan that, he hoped, would endear him back home to a nation exhausted and dispirited by years of conflict and corruption.

In exchange, he was prepared to offer up his country’s vast mineral wealth — unparalleled in much of the world.

In the imposing hall on Tiananmen Square, the two presidents outlined a deal that would change Central Africa’s balance of power, according to André Kapanga, a former adviser to Mr. Kabila who offered details of the meeting for the first time in an interview with The Times.

Mr. Hu explained that many people in China’s western provinces lived in deep poverty. Developing the area was a cornerstone of his domestic policy, and he needed minerals and metals to build out new industries. Congo was ready to help, Mr. Kabila assured him.

China had already acquired raw materials from Congo’s neighbor, Angola, where it offered generous financial support in exchange for oil.

But this potential deal with Mr. Kabila was more ambitious than any other, and a diplomatic drama would play out at the riverside Palais de la Nation in the capital of Kinshasa before it was sealed.

The setting was Mr. Kabila’s inauguration in 2006, after he stood before voters in a formal election and won the presidency. The Bush administration sent a delegation led by Elaine Chao, then the secretary of labor.

Mr. Kabila liked motorcycles, and she presented him with a Harley-Davidson trinket when she greeted him at a lunch. That would be the extent of their interaction, Ms. Chao believed, but members of her delegation urged her to ask for a private meeting, according to Laura Genero, an associate deputy labor secretary who was on the trip. To her surprise, Mr. Kabila complied with a meeting the next day.

Ms. Chao was so unprepared for the invitation that she had to borrow a beige pantsuit from Ms. Genero. She had packed just one work outfit.

The U.S. delegation congratulated Mr. Kabila on his democratic victory and listened as he talked about wanting to expand access to electricity across the nation. One of his aides characterized the meeting as mostly small talk.

But a similar meeting between the new president and Chinese officials played out differently, according to Mr. Kapanga, who was briefed on both the U.S. and Chinese discussions.

The Chinese used the opportunity to begin formal talks with Mr. Kabila that would result in a $6 billion agreement: China would pay for roads, hospitals, rail lines, schools and projects to expand electricity, all in exchange for access to 10 million tons of copper and more than 600,000 tons of cobalt.

The local media called it the “the deal of the century,” and while Mr. Kabila celebrated the agreement, the global financial community reacted more warily, worried Congo was taking on too much debt.

American officials marveled at the deal’s historic scale. In secret cables made public by WikiLeaks, they noted that previous Chinese investment in Congo had been “an informal, somewhat disorganized collection of Chinese businesses” that did not seriously threaten U.S. interests.

Now something much grander was in the making: “2,000 miles of roadway linking Orientale and Katanga provinces, 31 hospitals, 145 health centers, two large universities and 5,000 government housing units are pledged,” according to a cable in 2008 from the U.S. embassy in Kinshasa to members of the Central Intelligence Agency, the secretary of state and other officials.

“And that’s not all,” the cable continued.

Attracting a Phoenix

By 2015, China’s presence in Congo had become visible in numerous infrastructure projects: Soccer stadiums rose from the dust, roadways were expanded, work began on water treatment facilities.

But not all of its progress in cornering the cobalt market could be measured in brick and mortar. The Chinese ambassador at the time, Wang Tongqing, kicked off an American-style diplomatic blitz.

Mr. Wang threw out the jump ball that year at a Chinese corporate basketball tournament that drew Congolese spectators.

He gave out scholarships to Congolese students to study in China and was on hand when a Chinese organization donated plane tickets for a Congolese choir to tour his country. At one point, he offered $1 million for Ebola relief in Congo.

Mr. Wang’s activities coincided with the 2015 rollout of his country’s “Made in China 2025” policy, which detailed China’s plan to transform itself into a “manufacturing superpower” in 10 areas, including batteries for electric vehicles.

Almost instantly a tidal wave of government-backed capital poured into Chinese companies in Congo and elsewhere. Deals quickly followed.

That year, the state-owned China Nonferrous Metal Mining Group said it would partner with Congo’s state mining company, Gécamines, to develop the Deziwa site, then one of the largest copper and cobalt concessions in the country.

In 2017, Zijin Mining, a Chinese state-backed company with a slogan of “Harmony Begets Wealth,” raised almost $700 million from a sale of private shares to develop its Kolwezi mine.

Public statements about the deals signaled some of China’s ambition, but the history and scale of the effort have not been previously reported.

Corporate filings, including annual reports and bond prospectuses, examined by The Times show that the five biggest Chinese companies in Congo had been given at least $124 billion in credit lines for their global operations. All of the companies are state-owned or have significant minority stakes held by various levels of the Chinese government.

“Unlike the U.S., the Chinese government is always behind Chinese investors in Africa and more specifically in D.R.C.,” said Mr. Kapanga, the former adviser to Mr. Kabila.

The biggest deal came in April 2016, when China Molybdenum, a company whose biggest shareholders are a government-owned company and a reclusive billionaire, made its $2.65 billion offer to buy Tenke Fungurume, an American-owned mine atop one of the biggest cobalt reserves in the world.

There was one complication. Freeport-McMoRan had a Canadian partner that had the right of first offer to buy its stake. China Molybdenum’s solution was to have a Shanghai-based private equity firm buy out the partner, but even that deal relied on money from the Chinese government.

None of the $1.14 billion raised to buy the partner’s share came from private investors, company filings show. Instead, it came from Chinese state-controlled entities, including from bank loans guaranteed by China Molybdenum as well as cash brought to the deal through obscure shell companies controlled by government-owned banks, according to the filings.

The board of the private equity firm, commonly known as BHR, was dominated by Chinese members but also included three Americans: Devon Archer, a businessman who later was convicted of defrauding the Oglala Sioux tribe in a case still working through the legal system, and James Bulger, son of the former president of the Massachusetts State Senate.

Another was Hunter Biden, whose father was vice president at the time.

It is not clear if Mr. Biden, who had helped found the firm in 2013, was involved in the deal. Mr. Biden did not respond to requests for comment. A former member of the BHR board, who was not authorized to speak about internal business matters, said that none of the Americans had played a role and that the fees generated for the work had not been distributed to Mr. Biden or others. A spokesman for President Biden on Friday said he had not been made aware of his son’s connection to the sale.

How and why the firm had become involved was a mystery to the chief executive who negotiated the sale for Freeport-McMoRan’s Canadian-based partner, Lundin Mining.

“Were they a partner, their adviser or a financier? I don’t know,” said Paul Conibear, then Lundin’s chief executive.

An elaborate event under white tents in Kinshasa celebrated China’s new ownership in May 2017. Mr. Wang was there along with Chinese officials who had helped finance the purchase — and a host of Chinese government-affiliated bankers looking to make even more mining deals.

Within a few years, they would help orchestrate China Molybdenum’s purchase of Kisanfu, the huge untapped cobalt reserve, from the same American mining giant. Together the sales marked a changing of the guard in Congo as the United States abandoned its mining interests — a problem that now weighs on President Biden as he and his aides have come to realize the extent of China’s dominance in clean energy.

“The D.R.C. has a vast territory, rich natural resources and great investment potential,” Mr. Wang told the crowd. “A Chinese proverb says, ‘Build a beautiful nest to attract the phoenix.’”

‘Safety Is Just on Paper’

At first, the changes seemed almost trivial at Tenke Fungurume — a 24-hour operation that employs more than 7,000 across a landscape the size of Los Angeles marked by deep craters and dust kicked up by earth-moving vehicles.

The new Chinese managers showed up in shorts and sneakers, a shock to employees who had been required to wear steel-toed boots and safety goggles.

“We were like, ‘Oh, this is not possible,’” said Pierrot Kitobo Sambisaya, who worked as a metallurgist at the mine for a decade until 2019 and had grown accustomed to a stricter environment.

Soon, work anniversaries came and went with no recognition. Holiday parties where workers’ families were invited to tour the mine no longer took place. Dozens of janitor and driver jobs once held by Congolese citizens went to the Chinese.

That was just the start. Employees were concerned that the mine was also becoming more dangerous, according to interviews with workers in communities surrounding the mine, current and former safety inspectors, Congolese government officials and mining executives.

Workers climbed into acid tanks to conduct repairs without checking the air quality. Others drove bulldozers and other heavy equipment without training or did dangerous welding jobs without proper oversight.

Last year, a worker was sitting in his truck while it was being towed, and it flipped. The worker tried to jump to safety, but the truck landed on him and crushed him to death, according to an annual operations report from China Molybdenum.

All of it was an extreme departure from the company’s American predecessor, which had “zero tolerance” for risky activities and safety violations, according to Alfred Kiloko Makeba, the veteran safety supervisor, and 10 other current and former employees, managers and contractors.

Freeport-McMoRan, which had built the mine, had learned some hard lessons years before at its copper and gold mine in Indonesia, facing international protest over its dumping toxic mine waste into a river in the rainforest as well as violent conflicts over its operations there.

In Congo, the company had its own struggles as it moved to build Tenke Fungurume, displacing more than 1,500 residents in a haphazard process. But once the mine opened, it gained an unusual amount of respect for its commitment to worker safety, both among local officials and U.S. diplomats.

Worker safety is an issue at other industrial mines in Congo, but under Freeport, employees who violated rules were immediately disciplined or fired, safety officers said. Records examined by The Times show just one reported death among workers during the eight years Freeport-McMoRan ran the mine, although it repeatedly published accounts of near-fatal accidents as cautionary guides.

When safety inspectors discovered violations after China Molybdenum took over, they were sometimes told to overlook them, or offered bribes to do so, workers and supervisors said. And when they did try to enforce the rules, violence sometimes followed.

One safety officer said he was thrown to the ground by a worker he had called out for improperly using welding equipment. The man twisted his arm and broke his cellphone and work-issue camera.

An executive at Gécamines, the Congolese agency that is a minority shareholder in the mine, said employees had reported confrontations and safety problems to the agency’s board. Safety issues are now part of a broader review of China Molybdenum’s operations.

Mr. Zhou, the China Molybdenum spokesman, denied that any inspectors had been assaulted. The allegations, he suggested, were probably being fabricated by fired employees.

In a statement to The Times, he said the mine had “a robust occupational health and safety framework in place and continues to exercise its zero tolerance rules.” In fact, he said, “internal statistics” published in a company report this year showed that worker injuries had declined since the company took over.

But employees who said they had been repeatedly told not to report injuries believed the data was being fixed as part of a campaign to cover up rising hazards.

That suggestion, which The Times was not able to independently confirm and which China Molybdenum disputed, was crystallized for Mr. Makeba one evening last year when he received an urgent phone call. A worker at the mine had fallen from a high perch after not wearing the required safety harness, he said.

Mr. Makeba rushed to the site and was shocked to learn, he said, that the worker, who had broken his leg, had been taken to a private clinic instead of the mine’s.

Mr. Makeba said the employee told him that his supervisors had paid him to keep quiet so that it would not be reported to management, where it would show up on the company’s audited injury tally.

When Mr. Makeba alerted his own boss, he said, he was told to drop the matter.

Mr. Zhou rejected Mr. Makeba’s account, adding that “any form of cover-up in disclosures is against rules, and corporate values.”

But according to Mr. Makeba and another safety manager still working at the mine, labor conditions have become increasingly important to automakers sensitive to consumer and shareholder demands. So China Molybdenum, they said, has blocked them from reporting near fatalities and routinely ignored other injuries.

“Safety is just on paper now,” Mr. Makeba said.

Problems at Tenke Fungurume are not just limited to employees’ complaints inside the mine.

Freeport-McMoRan had struggled with trespassers who carted off bags of cobalt. Some even died when hand-dug tunnels flooded or collapsed.

With China Molybdenum in charge, the conflict became much worse.

The company, faced with thousands of newly arriving trespassers, asked the government to send soldiers to help control the situation, one executive who worked at the mine back then told The Times.

The military arrived and began patrolling Tenke Fungurume and other local mines, bulldozing depots where trespassers were selling their cobalt rocks to traders.

The troops remained for months, and the situation eventually turned deadly. A soldier at Tenke Fungurume opened fire, killing an unauthorized digger, according to an employee who told The Times he had witnessed the encounter.

Riots then erupted in the man’s home village when friends arrived carrying his body. In the melee, a protester was shot dead, according to three local officials and the mine employee.

China Molybdenum paid for the burials, they said.

Troops with AK-47s were posted outside the mine this year, along with security guards hired from a company founded by Erik Prince, the former Navy SEAL turned private security consultant.

Even as this crackdown on theft was underway, the new managers at the mine were looking for ways to cut costs while increasing production.

China Molybdenum said it had saved more than $130 million a year through its “cost and efficiency” programs. “New management revitalizes the business by bringing ‘Chinese efficiency and Chinese elements,’” the company boasts on its website.

The Rush to Expand

China Molybdenum is steadily growing its output. Last December, it snatched up Kisanfu, paying Freeport-McMoRan $550 million for what is considered one of the world’s largest untapped supplies of cobalt. The ground underneath the site contains enough cobalt, according to China Molybdenum’s estimates, to power hundreds of millions of long-range Teslas.

And then in August, China Molybdenum announced plans to spend $2.5 billion at Tenke Fungurume to double production over the next two years. When the expansion is complete, the mine will produce nearly 40,000 tons a year. Last year, the United States produced just 600 tons.

This rush to expand, however, has drawn scrutiny from top government officials in Congo, reaching all the way to Mr. Tshisekedi, the president.

Questions have surfaced over payments Tenke Fungurume’s operators may owe to Congo, dating back to when the American company controlled the mine. When new deposits are confirmed at Tenke Fungurume, the owners are required to notify Gécamines, the Congolese agency, and pay $12 for every additional ton.

The accusations have provoked a bitter dispute between Congolese officials and the mine managers, with China Molybdenum’s spokesman calling the allegations “unbelievable, wrong calculations” based on an accounting error.

Gécamines executives have discussed forcing out the management at Tenke Fungurume or even taking the mine out of China Molybdenum’s control, according to two Congolese mining executives involved in confidential discussions as well as a government official briefed on the talks.

Robert North, a New Mexico-based geologist who has helped prepare reserve estimates at the mine for Freeport and China Molybdenum, said both companies as well as Gécamines knew of large amounts of cobalt underground at the site. China Molybdenum has been cautious in declaring it, he said, until the company knows it wants to go to the expense of extracting the deeper layers.

Mr. Tshisekedi’s commission is still investigating the allegations, and the president himself recently presided over a tense, six-hour meeting with top company executives.

Separately, the Congolese government, with financial assistance from the United States, is examining numerous mining contracts to determine whether Congo has been shortchanged more broadly. While the Chinese-funded infrastructure projects got off to a flashy start, many have not been built, officials said.

During a visit to the cobalt-mining region this year, the president acknowledged that corrupt or incompetent government officials in Congo might deserve some blame for deals that have left the nation feeling shortchanged.

“Some of our compatriots had badly negotiated the mining contracts,” he said. “I’m very harsh on these investors who come to enrich themselves alone. They come with empty pockets and leave as billionaires.”

Chinese government officials insist that the relationship is still on track and that the benefits to Congo are substantial.

The countries have a “longstanding friendship, and the bilateral practical cooperation has yielded fruitful win-win results and enjoys broad prospects,” Zhao Lijian, spokesman for China’s Ministry of Foreign Affairs, said at a news conference in September.

In an interview in Kinshasa, Mr. Tshisekedi said that his focus was not on which foreign power would dominate mining in Congo, but rather on how his country could share in the wealth generated by the clean energy revolution.

“We have an amazing potential for renewable energy, be it through our strategic metals or through our rivers,” he said, referring to both mining and hydroelectric power. “Our idea is, how can we put this amazing resource at the disposal of the world, but while making sure that it first benefits Congolese and it benefits Africans?”

Dionne Searcey reported from Kisanfu, Michael Forsythe from New York and Eric Lipton from Washington. Keith Bradsher contributed reporting from Shanghai.

SAN JOSE, Calif. — For the past 11 weeks, prosecutors revealed emails from desperate investors. They held up falsified documents side-by-side with the originals. They called dozens of witnesses who lobbed accusations of deceit and evasiveness.

And on Friday, after questioning their 29th witness, prosecutors concluded their arguments against Elizabeth Holmes, the founder of the failed blood testing start-up Theranos. Ms. Holmes has pleaded not guilty to 11 counts of defrauding investors over Theranos’s technology and business, in a case that has been billed as a referendum on Silicon Valley’s start-up culture.

The prosecution’s resting its case is a major turning point in the trial of Ms. Holmes, whose rise and fall captivated the public and who has been held up as a symbol of the tech industry’s hubris and the last decade’s culture of grift.

For weeks, prosecutors sought to paint Ms. Holmes, 37, as a liar who built Theranos into a $9 billion start-up while knowing all along that the company’s blood tests, which were trumpeted as revolutionary, didn’t work. Prosecutors methodically outlined six main areas of Ms. Holmes’s deception, including lies about Theranos’s work with the military and pharmaceutical companies, its business performance and the accuracy of its blood tests.

Her lawyers are now expected to argue that Theranos was merely a failure and not a fraud, raising the question of whether Ms. Holmes will take the stand in her own defense. In filings, her lawyers have indicated that she is likely to testify.

The stakes of the trial are high. If Ms. Holmes is convicted, she faces up to 20 years in prison for each count of fraud, and prosecutors may be emboldened to go after more start-ups that stretch the truth to raise funding. An acquittal could send a message that Silicon Valley start-ups, which have exploded in power and wealth over the last decade, are difficult to hold to account.

“When prosecutors rest their case, they are basically saying they have enough to ask the jury to convict the defendant right then and there,” said Andrey Spektor, a lawyer at Bryan Cave Leighton Paisner and former federal prosecutor in New York’s Eastern District. He said he expected the defense to put on a case, rather than simply allow the jury to decide whether the prosecutors failed to prove theirs.

Ms. Holmes, a Stanford University dropout who founded Theranos in 2003 and raised $945 million from investors, was indicted on fraud charges in 2018. Her case has been plagued for years by delays: First over process, then the pandemic and finally, Ms. Holmes’s giving birth to a baby in August.

When the trial finally began in September, prosecutors called former investors, partners and Theranos employees to testify. Jim Mattis, the retired four-star Marine Corps general and former defense secretary who was a Theranos director, took the stand, as did a former Theranos lab director who endured six grueling days of questioning. In one surreal moment, a forensics expert recited text messages between Ms. Holmes and her then-boyfriend and business partner at Theranos, Ramesh Balwani, who is known as Sunny.

This week, Alan Eisenman, an early investor in Theranos, testified that Ms. Holmes cut him off and threatened him when he asked her for more information about the company. Yet even after that treatment, Mr. Eisenman poured more money into the start-up, believing its seemingly fast-growing business would deliver riches to backers like him.

When asked about his understanding of the value of his Theranos stock today, Mr. Eisenman said, “It’s not an understanding, it’s a conclusion. It’s worth zero.”

The prosecution’s most compelling evidence included a series of validation reports that Ms. Holmes sent to potential investors and partners that made it look as though pharmaceutical companies including Pfizer and Schering-Plough had endorsed Theranos’s technology. Representatives from each company testified that they had not endorsed Theranos’s blood test and were surprised to see their companies’ logos added to the report.

Daniel Edlin, who worked at Theranos and was a fraternity brother of Ms. Holmes’s brother, Christian, testified that the start-up faked demonstrations of its machines for potential investors, hid technology failures and threw out abnormal blood test results.

Mr. Mattis testified that he was not aware of any contracts between Theranos and the military to put its machines on medevac helicopters or on the battlefield, as Ms. Holmes had frequently told investors.

The prosecution concluded its case with testimony from Roger Parloff, the journalist who wrote a magazine cover story about Ms. Holmes, helping propel her to acclaim. Mr. Parloff’s article was sent to numerous investors as part of Ms. Holmes’s pitch.

Yet notably absent from the courtroom were some of the most prominent witnesses on the prosecution’s list. Ms. Holmes’s rise was aided by her association with business titans such as the media mogul Rupert Murdoch, elder statesmen such as Henry Kissinger and Admiral Gary Roughead, and the lawyer David Boies. Theranos was felled, in part, by whistle-blowers such as Tyler Shultz, the grandson of George Shultz, the former secretary of state who sat on Theranos’s board. None of them testified.

Also absent was Mr. Balwani, who was charged with fraud alongside Ms. Holmes and faces trial next year. His role as a fiery defender of Theranos who went after anyone who questioned the company has been in the background of much of the testimony.

At nearly every turn, Ms. Holmes’s lawyers sought to limit testimony and evidence. They attacked the credibility of investors, using legal disclaimers to show that investors knew they were gambling on a young start-up. The lawyers also poked holes in investors’ limited due diligence on Theranos’s claims. At one point, they directed Erika Cheung, a key whistle-blower who worked in Theranos’s lab, to read the entire organizational chart of the people employed in lab to show she played a small role in the overall operation.

The defense successfully argued to have one count of fraud against Ms. Holmes dropped. A patient who received a suspicious test from Theranos was barred from testifying earlier this month.

Ms. Holmes’s lawyers are likely to try to shine a spotlight on her relationship with Mr. Balwani. The two dated in secret. In court filings, Ms. Holmes alleged he was emotionally abusive and controlling. Mr. Balwani’s lawyers have denied the claims.

Testimony from Ms. Holmes is likely to revive the media circus surrounding the trial’s early days, which died down as the weeks of testimony wore on. It would also open her up to potentially damaging cross-examination from prosecutors or perjury.

“Most criminal defendants do not testify, particularly in white-collar cases where the government has many challenges to overcome, like in proving intent, and sometimes even in just proving that a crime occurred,” Mr. Spektor said. Ms. Holmes’s case is different, he said, because the offense is clear cut and the evidence is fairly easy to understand.

Throughout the proceedings, Ms. Holmes has been quiet in the courtroom, only whispering to her lawyers or family members. But the jury heard her forcefully defending Theranos against accusations of fraud in video interviews played in court. They also heard her accept blame.

“I’m the founder and C.E.O. of this company,” she said in one of the videos. “Anything that happens in this company is my responsibility.”