Special Counsel Robert Mueller has reportedly enlisted a grand jury to help investigate Russia's interference in the 2016 presidential election.
According to the Wall Street Journal, the move is a sign the investigation is intensifying and could go on for months.
It suggests Mueller is prepared to subpoena records and take witness testimony, but it does not necessarily mean he plans to bring charges against President Trump or other White House officials.
Mueller is looking into possible collusion between members of Trump's campaign and the Kremlin.
The president has denied any contact with Russia, and has repeatedly dismissed the investigation as a witch hunt.
His attorney told the Journal that the White House is fully committed to working with Mueller.