William Campbell, a Russian lobbyist, is the informant, according to Reuters. He will be testifying before a congressional committee about the 2010 sale of Uranium One, where a Russian-backed company bought a uranium firm with mines in the U.S. Campbell gave information to the FBI about what he saw while undercover as an informant.

Congressional committees have previously tried to interview Campbell, as he was undercover for roughly five years, working to get information on Russia's efforts to grow its atomic energy business in the U.S.