Rinat Akhmetshin, a former Soviet intel officer turned lobbyist, attended a meeting last year with senior aides to President Donald Trump and his son, it has emerged.
Rinat Akhmetshin confirmed to 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.
Donald Trump’s eldest son has only previously acknowledged that a Russian lawyer, Natalia Veselnitskaya, was present.
Donald Trump Jr. has played down the June 9, 2016, meeting since it was reported this week.
However, the Senate Judiciary Committee has asked Donald Trump Jr. to publicly testify.
Several congressional panels and federal investigators are looking into claims that Russia meddled in last year’s election in a bid to help Donald Trump.
The New York meeting was also attended by Donald Trump’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, President Trump said that “most people would have taken that meeting”.
“Zero happened,” he said.
Rinat Akhmetshin told the AP news agency he served in a Soviet military unit that was part of counterintelligence, but that he was never formally trained as a spy.
In court papers filed with the US District Court in Washington DC, where he lives, a mining company accused him of organizing a hack on its private records as part of an alleged smear campaign.
International Mineral Resources hired a private investigator to follow Rinat Akhmetshin to London.
The Russian-American was overheard in a coffee shop bragging about arranging the cyber-attack on the firm’s computer system, according to court documents.
Rinat Akhmetshin denied the allegations, and the case was later dropped.
Kremlin spokesman Dmitry Peskov says the Russian government knows nothing about Rinat Akhmetshin, the AP reported.
Rinat Akhmetshin told the Washington Post he became a US citizen in 2009 but retains his Russian nationality.
He is a registered lobbyist who has focused in recent years on overturning the 2012 US Magnitsky Act.
The law, which froze the assets of senior Russian officials, angered President Vladimir Putin.
Rinat Akhmetshin told the Washington Post he only accompanied Natalia Veselnitskaya to the Trump Tower meeting on the spur of the moment after meeting her for lunch.
He said he showed up with her at the Manhattan skyscraper wearing jeans and a T-shirt.
Rinat Akhmetshin said he and Natalia Veselnitskaya were joined by an interpreter.
“I never thought this would be such a big deal, to be honest,” he said.
The meeting was set up by Rob Goldstone, a British music publicist for Russian pop star Emin Agalarov.
Emin Agalarov worked with Donald Trump on staging the Miss Universe pageant in Moscow in 2013.
Emails show that Rob Goldstone promised Donald Trump Jr. that Natalia Veselnitskaya, whom he described as a “Russian government attorney”, would share damaging information about Hillary Clinton.
Donald Trump Jr. said it became apparent as they spoke that Natalia Veselnitskaya wanted to discuss a suspended program for Americans to adopt Russian children.
However, Rinat Akhmetshin told the AP that Natalia Veselnitskaya informed Donald Trump Jr. she had details of possible illicit funds being funneled to the Democratic National Committee (DNC).
“This could be a good issue to expose how the DNC is accepting bad money,” he said he recalled her saying.
According to Rinat Akhmetshin, Donald Trump Jr. asked her if she had evidence to prove the claim, but the lawyer urged the Trump campaign to research the matter.
Donald Trump’s son appeared to lose interest, said Rinat Akhmetshin.
“They couldn’t wait for the meeting to end,” he added.