In 2010, Rosatom, the Russian nuclear energy agency, bought a controlling interest in Uranium One, a Canadian-based company that owned valuable extraction rights in Kazakhstan, Canada and Australia. These three countries are the world’s largest uranium producers, and Russia’s acquisition of those rights served its ambitions to build and fuel nuclear power plants around the world.

In an ancillary part of the deal, Rosatom also bought mining rights in the United States, where uranium production is sparse. These rights were initially estimated to represent 20 percent of American production, but in practice, this figure has averaged less than 10 percent. Overall, Rosatom’s purchase of Uranium One transferred control of what now amounts to about 8 percent of world uranium output. The American portion represented about one-thirtieth of the deal.

At the time, none of this was considered remarkable or controversial. The governments of Kazakhstan, Canada and Australia approved the transfer as a normal commercial transaction. Similarly, the United States’ part of the deal was authorized by a panel of officials from nine federal agencies — including the State Department, then led by Hillary Clinton.

But in 2016, in the run-up to the presidential campaign, the sale became notorious. A growing chorus of Trump supporters claimed that the sale didn’t happen for business reasons, but in exchange for millions in donations by people associated with Uranium One to the Clinton Foundation, led by Mrs. Clinton’s husband, former President Bill Clinton.