A grand jury used by special counsel Robert Mueller has heard secret testimony from a Russian-American lobbyist who attended a June 2016 meeting with President Donald Trump's eldest son, a person familiar with the matter told the Associated Press on Wednesday.

That former Soviet army officer, Rinat Akhmetshin, appeared before Mueller's grand jury in recent weeks, said the source, who spoke on condition of anonymity.

Attendees at the 2016 meeting in Trump Tower in New York City included Donald Trump Jr.; the president's son-in-law, Jared Kushner; and President Trump's former campaign chairman, Paul Manafort.

Emails released by Trump Jr. show he took the meeting expecting to receive damaging information about presidential candidate Hillary Clinton as part of what was described to him as a Russian government effort to aid the Trump campaign.

The revelation, which was first reported in the Financial Times, is the clearest indication yet that Mueller and his team of investigators view the meeting, which came weeks after Trump had secured the Republican presidential nomination, as a relevant inquiry point in their broader probe into Russian interference in the 2016 election.

“This is obviously very high-level and sensitive information but is part of Russia and its government’s support for Mr. Trump,” music publicist Rob Goldstone, who helped set up the meeting, wrote in an email to Trump Jr., according to the Hill. He also assured the president’s son that Russian lawyer Natalia Veselnitskaya, who also attended the meeting, had information that “would incriminate Hillary and her dealings with Russia and would be very useful,” to then candidate Trump.

“If it’s what you say I love it especially in the summer,” Trump Jr. replied.

Trump Jr. — who is set to testify before the Senate Judiciary Committee sometime over the next few weeks — has denied any wrongdoing in taking the meeting.

Akhmetshin, a former Soviet military officer who served in a counterintelligence unit, is also a well-known Washington lobbyist. He has been representing Russian interests trying to undermine the story of lawyer Sergei Magnitsky, who died in a Russian prison and is the namesake of a U.S. sanctions law.

Akhmetshin has been reported to have ties to Russian intelligence but he has denied that, calling the allegations a "smear campaign."

Mueller and his team first signaled their interest in the Trump Tower gathering last month by contacting an attorney for at least some of the Russians who attended.

The Associated Press contributed to this story.