According to the Financial Times, Rinat Akhmetshin, one of the Russians who met with Donald Trump Jr. during the campaign at Trump Tower with promises of evidence incriminating Trump’s opponent Hillary Clinton, testified to Mueller’s grand jury in Washington for several hours on August 11th. The article linked above includes Mr. Akhmetshin’s comments to the Financial Times and a summery of what is known about the meeting publicly.
While naturally no details were provided about the testimony, this could be a sign that Mr. Mueller’s investigation is looking seriously into the meeting in 2016, which included Trump’s son-in-law Jared Kushner and Russian lawyer Natalia Veselnitskaya in addition to Mr. Trump and Mr. Akhmetshin.