A lobbyist who was once a Soviet counter-intelligence officer participated last year in a meeting with senior aides to US President Donald Trump, including his son, and a Russian lawyer, NBC News reports.
The news adds to allegations of possible connections between Moscow and the November election.
NBC News, which did not identify the Russian-American lobbyist, said some US officials suspected him of having ongoing ties to Russian intelligence, something he denied to the network.
The Associated Press said the lobbyist, whom it identified as Rinat Akhmetshin, confirmed that he had attended the June 2016 meeting in New York's Trump Tower.
Despite intense media focus on the meeting, his presence had not been reported or disclosed until Friday.
Emails disclosed by Donald Trump Jr this week showed him eager to attend the meeting with the lawyer, Natalia Veselnitskaya, after learning that she might have information damaging to Democratic presidential candidate Hillary Clinton.
Last weekend's revelation of the Trump Tower gathering would be the most tangible evidence of a connection between Mr Trump's 2016 campaign and Russia, a subject that has also prompted investigations by congressional committees and federal special counsel.
Mr Trump's son-in-law, Jared Kushner, and former campaign adviser Paul Manafort were also at the meeting.
Accusations that Moscow meddled in the election and colluded with the Trump campaign have dominated the Republican president's first months in office. Russia denies the allegations, and Mr Trump says there was no collusion.
US intelligence agencies said earlier this year that Russia sought to help Mr Trump win the election by hacking private emails from Democratic Party officials and disseminating false information online.
NBC News said the Russian-born American lobbyist who attended the meeting with Trump aides had served in the Soviet military and emigrated to the US, where he holds dual citizenship. He denied any current ties to Russian spy agencies.
NBC News said a lawyer for Mr Trump Jr said he had spoken to the lobbyist. Mr Trump Jr's lawyer, Alan Futerfas, said he had talked with the person who came to the meeting with Ms Veselnitskaya.
"He is a US citizen," Mr Futerfas told NBC News. "He told me specifically he was not working for the Russian government, and in fact laughed when I asked him that question."
The lawyer said Mr Trump Jr knew nothing about the man's background at the time of the meeting.