A former Soviet counter-intelligence official attended a meeting last year with senior aides to President Donald Trump and his son, it has emerged, BBC reports.
Rinat Akhmetshin, now a lobbyist, confirmed to US media he was present at the Trump Tower encounter.
Donald Trump Jr was promised damaging material on Hillary Clinton at the meeting, his emails show.
Trump Jr has only previously acknowledged that a Russian lawyer, Natalia Veselnitskaya, was present.
The president's eldest son has played down the 9 June 2016 meeting since it was reported this week.
But the Senate Judiciary Committee has asked 39-year-old Trump Jr to publicly testify.
Several congressional panels and federal investigators are looking into claims that Russia meddled in last year's US election in a bid to help Donald Trump.
Media captionDonald Trump Jr maintains that there was nothing to tell his father President Trump
The New York meeting was also attended by the president's son-in-law, Jared Kushner, and election chairman, Paul Manafort.
It has emerged as the firmest evidence yet of non-diplomatic interactions between Trump campaign aides and Russian figures.
During a news conference in Paris, France, on Thursday, Trump said that "most people would have taken that meeting".
"Zero happened," the president said.