The former manager of Donald Trump's presidential campaign, Paul Manafort, is to face charges related to an inquiry into alleged Russian election meddling, US media report.
He was seen arriving at an FBI office in Washington with his lawyer early on Monday.
An investigation headed by special counsel Robert Mueller is looking into any links between Russia and the Trump campaign.
Both sides deny any collusion.
Mr Mueller's team is known to have conducted extensive interviews with several current and former White House officials as part of the inquiry.
One of Mr Manafort's business associates, Rick Gates, has also been ordered to surrender to authorities, according to the reports.
The nature of the charges Mr Manafort and Mr Gates face is not yet clear.
Mr Manafort, 68, has worked on several Republican presidential campaigns, beginning with Gerald Ford's in 1976.
He resigned as chairman of the Trump campaign in August 2016 after being accused of having links to a pro-Russian political party working in Ukraine. He denies any wrongdoing.
Mr Manafort was also present at a meeting with a Russian lawyer who promised damaging information on Mr Trump's rival Hillary Clinton.
US intelligence agencies have already concluded that the Russian government sought to help Mr Trump win the election.
But President Trump has strenuously rejected allegations of collusion, calling the investigation a "witch hunt".