A former Soviet counter-intelligence official attended a meeting last year with senior aides to President Donald Trump and his son, it has emerged.
Rinat Akhmetshin, now a lobbyist, told AP news agency he was present at the encounter in Trump Tower.
Donald Trump Jr was promised damaging material on Hillary Clinton at the meeting, his emails show.
Mr 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 the president's son to publicly testify.
It is one of several congressional panels that - along with federal investigators - is looking into claims that Russia meddled in last year's US election in a bid to help Donald Trump.
Also present at the Trump Tower meeting were the president's son-in-law, Jared Kushner, and campaign chairman Paul Manafort.
During a news conference in Paris on Thursday, Mr Trump defended his son, saying he was merely attempting to collect opposition research and that "most people would have taken that meeting".
"Zero happened," Mr Trump said.
Who is Rinat Akhmetshin?
Mr Akhmetshin told the AP he served in a Soviet military unit that was part of counterintelligence, but was never formally trained as a spy.
In court papers filed with the US District Court in Washington DC, a mining company accused him of organising a hack on the company's private records as part of an alleged smear campaign.
International Mineral Resources (IMR) hired a private investigator to follow Mr Akhmetshin to London.
The Russian-American was overheard bragging about arranging the cyber-attack on IMR's computer system, according to court documents.
Mr Akhmetshin denied the allegations, and the case was later dropped.
Kremlin spokesman Dmitry Peskov told reporters the Kremlin knows nothing about Mr Akhmetshin, the AP reported.
Why was Akhmetshin at the meeting?
Mr Akhmetshin told the AP he had only learned of the Trump Tower meeting on the same day he accompanied Ms Veselnitskaya to the Manhattan skyscraper, adding that he showed up in jeans and a t-shirt.
He said he and Ms Veselnitskaya met an interpreter who also participated in the encounter.
"I never thought this would be such a big deal, to be honest," he said.
Mr Trump Jr's lawyer, Alan Futerfas, told the Hill, a Washington DC politics news outlet, that the lobbyist had been introduced during the meeting as a friend of Ms Veselnitskaya.
The lawyer said: "He's a US citizen. All I can say is Trump Jr met him for the first time and learned his name minutes before the meeting and he was introduced as a friend."
The meeting was set up by Rob Goldstone, a British music publicist for Russian pop star Emin Agalarov.
Mr Agalarov worked with Donald Trump Snr on staging the Miss Universe pageant in Moscow in 2013.
What was the meeting about?
Emails show that Mr Goldstone promised Mr Trump Jr that Ms Veselnitskaya would share damaging information at the face-to-face about Mrs Clinton, Mr Trump's Democratic party rival for the White House.
Mr Trump Jr said it later became apparent that Ms Veselnitskaya wanted to discuss a suspended programme for Americans to adopt Russian children.
Why was this British guy emailing Trump Jr?
But Mr Akhmetshin told the AP that Ms Veselnitskaya told Mr Trump Jr she had details of what she believed were illicit funds being funnelled to the Democratic National Committee.
"This could be a good issue to expose how the DNC is accepting bad money," he said he recalled her saying.
According to Mr Akhmetshin, Mr Trump Jr asked her if she had evidence to prove the claim, but the lawyer said the Trump campaign would need to do more research.
The president's son appeared to lose interest, said Mr Akhmetshin.
"They couldn't wait for the meeting to end," he added.