The former federal informant at the center of the Senate GOP probe into former Secretary of State Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE's role in the Obama-era sale of a uranium company was identified Thursday as an ex-lobbyist for a Russian firm.

The former lobbyist, William Campbell, told Reuters he's the secret witness in the investigation into the 2010 sale of Canada-based Uranium One to Russia's Rosatom. He is planning to testify and give Congress documents about the sale, he told Reuters.

Despite the focus on Campbell and calls for him to be allowed to testify to Congress, two law enforcement officials speaking to Reuters expressed doubt that Campbell could provide meaningful information about the Uranium One deal. The officials, who worked in the Rosatom bribery case, said they don't remember Campbell ever bringing up the Uranium One sale when they spoke with him.

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According to the publication, Campbell worked as an FBI source for an investigation into the head of a U.S. unit of Rosatom, the Russian state-owned nuclear power company.

Campbell reportedly told Reuters he wants to testify, citing activities of Russians in the U.S.

"This latest iteration is simply more of the Right doing Trump's bidding for him to distract from his own Russia problems," said Clinton spokesman Nick Merrill.