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.