President Trump's ex-campaign chairman Paul Manafort and his former business associate Rick Gates surrendered to federal authorities early Monday in special counsel Robert Mueller's first changes related to the 2016 Russia investigation.

The charges against the two were not immediately clear, but the indictment was expected to be unsealed Monday. The Wall Street Journal reported that charges against Manafort include tax fraud.

Manafort and his lawyer were seen walking into the FBI's field office in Washington early Monday. He was also captured by photographers leaving his home in Alexandria, Va., early Monday with his lawyer. That same home was raided by the FBI in July, and agents reportedly seized financial and tax documents.

Paul Manafort arrives at FBI field office in DC pic.twitter.com/TQHOZ4tb20— Washington Examiner (@dcexaminer) October 30, 2017



Reports over the weekend indicated that Mueller was close to an indictment in his investigation.

Related: Related: Manafort, Gates indicted on 12 counts, including 'conspiracy against United States'

Read the indictment against Paul Manafort and Rick Gates

The FBI was reportedly focusing on Manafort because of suspicious wire transfers to offshore companies linked to him.

The FBI may be looking into whether Manafort committed tax fraud or was laundering money for Ukrainian officials.

Those charges go beyond Mueller's probe into Russian campaign meddling, and Trump himself has argued that Mueller's probe is a political " witch hunt."

More: Trump lawyer says the president's tweets unrelated to Russia probe charges being issued

IRS shares info on Manafort with Mueller

Millions in suspicious wire transfers are the focus of FBI's probe into Paul Manafort

According to the Times' report, Gates is “a longtime protégé" of Manafort, and his name appears on documents linking Manafort’s firm “set up in Cyprus to receive payments from politicians and businesspeople in Eastern Europe.”

Manafort joined the Trump campaign in March 2016, and served as campaign manager from June to August 2016.

Mueller reports to Deputy Attorney General Rod Rosenstein, who took over the Russia investigation after Attorney General Jeff Sessions recused himself. That move drew complaints from Trump, who has repeatedly called for the investigation to conclude. The president and his associates have all claimed no wrongdoing.