President Donald Trumps former campaign manager Paul Manafort surrendered to the FBI on Monday and another ex-aide pleaded guilty to lying to agents in the most serious steps yet of a federal probe into Russian interference in the 2016 presidential election. Manafort, 68, a longtime Republican operative, arrived at the Federal Bureau of Investigation’s Washington field office to hand himself in after being indicted by a federal grand jury on charges including money laundering and conspiracy against the United States. In a separate announcement on Monday, the office of Justice Department Special Counsel Robert Mueller said former Trump campaign aide George Papadopoulos had pleaded guilty on Oct. 5 to making false statements to FBI agents in the Russia probe. Papadopoulos is an international energy lawyer. According to Reuters, Manafort associate Rick Gates was named alongside Manafort in the 12-count indictment, which was the first from Mueller’s investigation into alleged Russian interference in last year’s campaign to try to tilt the vote in Trump’s favor. Neither Trump nor his campaign were mentioned in the indictment and many of the charges, some of which go back more than a decade, appear related to Manafort’s work for Ukraine’s pro-Russian government and political figures there. Russia investigations by Mueller and several congressional panels have cast a shadow over the Republican president’s first nine months in office and have widened the rift between Republicans and Democrats. Manafort ran the Trump campaign from June to August of 2016 before resigning amid reports he might have received millions in illegal payments from a pro-Russian political party in Ukraine. Lawyers for Gates and Manafort did not immediately return calls for comment. An initial court appearance and arraignment for the two men was scheduled for 1:30 p.m. on Monday in Washington before Magistrate Judge Deborah Robinson, a court spokeswoman said. Trump, who has denied any allegations of collusion with the Russians, reiterated on Monday his public frustration with the Mueller probe, which he has called “a witch hunt.” “Sorry, but this is years ago, before Paul Manafort was part of the Trump campaign. But why aren’t Crooked Hillary & the Dems the focus?????,” Trump said in a Twitter post on Monday morning, referring to his Democratic presidential rival last year, Hillary Clinton. Legal experts said the indictment could just be an opening salvo by Mueller. It put pressure on Manafort to cooperate with Mueller’s Russia investigation, said Renato Mariotti, a former federal prosecutor in Chicago. “If I were the defense lawyer I’d be looking into cooperating,” he said.