Paul Manafort, President Trump’s former campaign chairman, surrendered himself to federal law enforcement authorities on Monday morning, a Justice Department source tells NPR’s Ryan Lucas, representing a new phase of the investigations into figures from Trump’s inner circle.

Both Manafort and Rick Gates, his former deputy, were told to surrender, The New York Times reports. Over the summer, the pair registered with the U.S. government as foreign agents.

Manafort was taken into federal custody days after news emerged that a federal grand jury assembled by Department of Justice special counsel Robert Mueller had voted to indict a figure in Mueller’s investigation into possible coordination between Russia and people close to the Trump campaign in last year’s presidential election.

The grand jury’s indictment may be unsealed on Monday; in addition to the main focus on Russia’s efforts to tamper with the U.S. democratic process, Mueller’s mandate also allows his team to probe any other criminal matters that arise during the course of its investigation.

FBI agents raided Manafort’s home in Alexandria, Va., over the summer.