Lawyer Natalia Veselnitskaya attended a meeting with Donald Trump Jr during the election campaign in June 2016 at which it appears he expected to get incriminating information from the Russian government on his father's rival Hillary Clinton.
Trump Jr had been told by email that a "Russian government attorney" would provide damaging details. Trump campaign manager Paul Manafort and son-in-law Jared Kushner also attended.
Veselnitskaya has rejected claims that she was working on behalf of the Russian government, while Trump Jr insists that no sensitive information was passed on.
The firestorm around the meeting is the latest chapter in the scandal over possible ties between the Trump campaign and Moscow.
US intelligence agencies have accused Russian President Vladimir Putin of approving a hacking and influence campaign to sway the November 2016 vote, allegations rejected by the Kremlin.
Trump's administration is currently under investigation by both Congress and the FBI over any possible collusion.