WASHINGTON (AP) — 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, The Associated Press has learned.

A person familiar with the matter confirmed to the AP that Rinat Akhmetshin had appeared before Mueller's grand jury in recent weeks. The person spoke on condition of anonymity to discuss the secret proceedings.

The revelation 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.

The meeting included Donald Trump Jr.; the president's son-in-law, Jared Kushner; and his former campaign chairman, Paul Manafort. Emails released by Trump Jr. show he took the meeting expecting that he would be receiving damaging information about Hillary Clinton as part of what was described to him as a Russian government effort to aid the Trump campaign.

The Financial Times first reported Akhmetshin's grand jury appearance. Reached by the AP, Akhmetshin declined comment. Peter Carr, a spokesman for Mueller, also declined comment Wednesday night.

The confirmation of Akhmetshin's grand jury testimony comes after he spoke at length about his involvement in the Trump Tower meeting in an interview with the AP last month.

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 meeting at issue was disclosed earlier this year to Congress and first revealed by The New York Times.

Trump Jr. has offered evolving explanations for the circumstances of the meeting, initially saying that the purpose was to discuss adoption and later acknowledging that he anticipated receiving information that he thought could be damaging to Clinton.

In addition to Akhmetshin, other attendees at the meeting included Russian lawyer Natalia Veselnitskaya, music publicist Rob Goldstone — who helped arrange the gathering — and a translator. Ike Kaveladze, who also goes by the name Irakly Kaveladze, also attended the meeting. Kaveladze works for a Russian developer who partnered with Trump on the 2013 Miss Universe pageant in Moscow.

An email exchange posted to Twitter by Trump Jr. showed him conversing with Goldstone, who wanted him to meet with someone he described as a "Russian government attorney," who supposedly had dirt on Clinton as "part of Russia and its government's support for Mr. Trump."

"If it's what you say I love it especially later in the summer," Trump Jr. wrote in one email response.

Another contact between Trump associates and Russia was revealed this week when Trump's personal lawyer, Michael Cohen, acknowledged that the Trump Organization was pursuing a Trump Tower real estate complex in Moscow in 2015. Cohen said he had reached out to a press secretary for Russian President Vladimir Putin about approvals.

In a letter this month to the House intelligence committee, Stephen Ryan, a lawyer for Cohen, dismissed as "false" and "wholly unsubstantiated" claims about Cohen included in a dossier of salacious allegations about the president's connections with Russia.

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Associated Press writer Desmond Butler contributed to this report.