President Donald Trump's onetime campaign chairman, Paul Manafort was indicted on 12 counts related to hiding foreign payments, including one count of "conspiracy against the United States."

Manafort's former longtime business partner, Rick Gates, also was charged in the 31 page indictment, made public on Monday. Gates and Manafort both surrendered to authorities on Monday morning.

The two men pleaded not guilty to all charges during a court appearance later Monday. A spokesman for Gates said the former lobbyist "welcomes the opportunity to confront these charges in court." A Manafort spokesman declined to comment.

In a statement, Special Counsel's Office spokesman Peter Carr said the indictment "contains 12 counts: conspiracy against the United States, conspiracy to launder money, unregistered agent of a foreign [government], false and misleading [foreign lobbying disclosures], false statements, and seven counts of failure to file reports of foreign bank and financial accounts. The case was unsealed today after the defendants were permitted to surrender themselves to the custody of the FBI."

Multiple law enforcement sources also told NBC News there was a statute of limitations issue in play that may have helped drive the timing of the indictments, which were handed down Friday.

Following news of the indictments, a White House source told NBC News that the charges had not come as a surprise to the Trump administration. "The White House has been saying for weeks the special counsel is moving far more quickly and deliberatively than people have been reporting. The fact that the special counsel is actively performing its duties does not come as a surprise to the White House," the source said.

The indictments mark a new phase in Special Counsel Robert Mueller's investigation into Russian attempts to interfere in the 2016 presidential election. They also cap off a year of escalating legal problems for Manafort and Gates, who worked with Manafort for years on many of his foreign lobbying contracts.