Senator John Barrasso, a Republican from Wyoming, where Uranium One's largest American operation was, wrote to President Obama, saying the deal "would give the Russian government control over a sizable portion of America's uranium production capacity."
"Equally alarming," Mr. Barrasso added, "this sale gives ARMZ a significant stake in uranium mines in Kazakhstan."
Uranium One's shareholders were also alarmed, and were "afraid of Rosatom as a Russian state giant," Sergei Novikov, a company spokesman, recalled in an interview. He said Rosatom's chief, Mr. Kiriyenko, sought to reassure Uranium One investors, promising that Rosatom would not break up the company and would keep the same management, including Mr. Telfer, the chairman. Another Rosatom official said publicly that it did not intend to increase its investment beyond 51 percent, and that it envisioned keeping Uranium One a public company.
American nuclear officials, too, seemed eager to assuage fears. The Nuclear Regulatory Commission wrote to Senator Barrasso assuring him that American uranium would be preserved for domestic use, regardless of who owned it.
"In order to export uranium from the United States, Uranium One Inc. or ARMZ would need to apply for and obtain a specific NRC license authorizing the export of uranium for use reactor fuel," the letter said.
Still, the ultimate authority to approve or reject the Russian acquisition rested with the cabinet officials on the foreign investment committee, including Mrs. Clinton — whose husband was collecting millions of dollars in donations from people associated with Uranium One.
Before Mrs. Clinton could assume her post as secretary of state, the White House demanded that she sign a memorandum of understanding placing limits on her husband's foundation's activities. To avoid the perception of conflicts of interest, beyond the ban on foreign government donations, the foundation was required to publicly disclose all contributors.
To judge from those disclosures — which list the contributions in ranges rather than precise amounts — the only Uranium One official to give to the Clinton Foundation was Mr. Telfer, the chairman, and the amount was relatively small: no more than $250,000, and that was in 2007, before talk of a Rosatom deal began percolating.
But a review of tax records in Canada, where Mr. Telfer has a family charity called the Fernwood Foundation, shows that he donated millions of dollars more, during and after the critical time when the foreign investment committee was reviewing his deal with the Russians. With the Russians offering a special dividend, shareholders like Mr. Telfer stood to profit.
His donations through the Fernwood Foundation included $1 million reported in 2009, the year his company appealed to the American Embassy to help it keep its mines in Kazakhstan; $250,000 in 2010, the year the Russians sought majority control; as well as $600,000 in 2011; and $500,000 in 2012. Mr. Telfer said that his donations had nothing to do with his business dealings, and that he had never discussed Uranium One with Mr. or Mrs. Clinton. He said he had given the money because he wanted to support Mr. Giustra's charitable endeavors with Mr. Clinton. "Frank and I have been friends and business partners for almost 20 years," he said.
The Clinton campaign left it to the foundation to reply to questions about the Fernwood donations; the foundation did not provide a response.
Mr. Telfer's undisclosed donations came in addition to between $1.3 million and $5.6 million in contributions, which were reported, from a constellation of people with ties to Uranium One or UrAsia, the company that originally acquired Uranium One's most valuable asset: the Kazakhstan mines. Without those assets, the Russians would have had no interest in the deal: "It wasn't the goal to buy the Wyoming mines. The goal was to acquire the Kazakh assets, which are very good," Mr. Novikov, the Rosatom spokesman, said in an interview.
Amid this influx of Uranium One-connected money, Mr. Clinton was invited to speak in Moscow in June 2010, the same month Rosatom struck its deal for a majority stake in Uranium One.
The $500,000 fee — among Mr. Clinton's highest — was paid by Renaissance Capital, a Russian investment bank with ties to the Kremlin that has invited world leaders, including Tony Blair, the former British prime minister, to speak at its annual investor conference.
Renaissance Capital analysts talked up Uranium One's stock, assigning it a "buy" rating and saying in a July 2010 research report that it was "the best play" in the uranium markets. In addition, Renaissance Capital turned up that same year as a major donor, along with Mr. Telfer and Mr. Giustra, to a small medical charity in Colorado run by a friend of Mr. Giustra's. In a newsletter to supporters, the friend credited Mr. Giustra with helping get donations from "businesses around the world."
A Renaissance Capital representative would not comment on the genesis of Mr. Clinton's speech to an audience that included leading Russian officials, or on whether it was connected to the Rosatom deal. According to a Russian government news service, Mr. Putin personally thanked Mr. Clinton for speaking.
A person with knowledge of the Clinton Foundation's fund-raising operation, who requested anonymity to speak candidly about it, said that for many people, the hope is that money will in fact buy influence: "Why do you think they are doing it — because they love them?" But whether it actually does is another question. And in this case, there were broader geopolitical pressures that likely came into play as the United States considered whether to approve the Rosatom-Uranium One deal.
If doing business with Rosatom was good for those involved with the Uranium One deal, engaging with Russia was also a priority of the incoming Obama administration, which was hoping for a new era of cooperation as Mr. Putin relinquished the presidency — if only for a term — to Dmitri A. Medvedev.
"The assumption was we could engage Russia to further core U.S. national security interests," said Mr. McFaul, the former ambassador.
It started out well. The two countries made progress on nuclear proliferation issues, and expanded use of Russian territory to resupply American forces in Afghanistan. Keeping Iran from obtaining a nuclear weapon was among the United States' top priorities, and in June 2010 Russia signed off on a United Nations resolution imposing tough new sanctions on that country.
Two months later, the deal giving ARMZ a controlling stake in Uranium One was submitted to the Committee on Foreign Investment in the United States for review. Because of the secrecy surrounding the process, it is hard to know whether the participants weighed the desire to improve bilateral relations against the potential risks of allowing the Russian government to control over the biggest uranium producer in the United States The deal was ultimately approved in October, following what two people involved in securing the approval said had been a relatively smooth process.
Not all of the committee's decisions are personally debated by the agency heads themselves; in less controversial cases, deputy or assistant secretaries may sign off. But experts and former committee members say Russia's interest in Uranium One and its American uranium reserves seemed to warrant attention at the highest levels.
"This deal had generated press, it had captured the attention of Congress and it was strategically important," said Richard Russell, who served on the committee during the George W. Bush administration. "When I was there invariably any one of those conditions would cause this to get pushed way up the chain, and here you had all three."
And Mrs. Clinton brought a reputation for hawkishness to the process; as a senator, she was a vocal critic of the committee's approval of a deal that would have transferred the management of major American seaports to a company based in the United Arab Emirates, and as a presidential candidate she had advocated legislation to strengthen the process.