Paul Manafort has been indicted by special counsel Robert Mueller and told to surrender

President Donald Trump's campaign chairman Paul Manafort and Manafort's former business associate Rick Gates were indicted by special counsel Robert Mueller and told to surrender to federal authorities on Monday, multiple media outlets have reported.

Manafort has been at the center of Mueller's investigation into whether the Trump campaign colluded with Moscow during the 2016 election. Legal experts speculated over the weekend, after news of the sealed indictments broke on Friday, that Mueller would arrest Manafort in an effort to get him to "flip" against Trump.

The FBI conducted a predawn July raid on Manafort's home. Manafort was told at the time that he would be indicted, according to his longtime friend Roger Stone. Investigators were looking for tax documents and foreign banking records, documents typically sought when investigating violations of Bank Secrecy Act.

The Bank Secrecy Act was passed "to deter and detect money laundering, terrorist financing, and other criminal acts and the misuse of our nation's financial institutions," according to the Treasury Department.

Manafort has been cooperating with investigators' requests for relevant documents. But the search warrant obtained by the FBI in July indicates that Mueller managed to convince a federal judge that Manafort would try to conceal or destroy documents subpoenaed by a grand jury.

The indictments isued on Friday were sealed, and Manafort's attorneys did not receive a target letter, raising similar questions about whether Mueller thought Manafort would have tried to flee or destroy evidence if he were notified of his impending arrest three days beforehand.

Legal experts saw the July raid as a sign that something very serious was coming.

A former Department of Justice spokesman, Matthew Miller, said a raid coming months into an investigation when the subject's attorneys had been speaking with, and presumably cooperating with, the DOJ "suggests something serious."

"Manafort's representatives have been insisting for months that he is cooperating with these investigations, and if you are really cooperating, DOJ typically doesn't need to raid your house - they'll trust you to respond fully to a subpoena," former Department of Justice spokesman Matthew Miller said at the time.

Manafort's ties to Russia came under scrutiny last August, when The New York Times discovered that a pro-Russian political party in Ukraine designated him $12.7 million in undisclosed cash payments. Manafort, a longtime Republican operative, had advised the party and its former leader Viktor Yanukovych for nearly a decade.

Manafort has been associated with at least 15 bank accounts and 10 companies in Cyprus, dating back to 2007, NBC reported in March, and the FBI has issued grand jury subpoenas to several banks for Manafort's records.

Manafort also attended a meeting last year at Trump Tower with Trump's son Donald Trump Jr., son-in-law Jared Kushner, and two Russian lobbyists who were said to have offered dirt on Hillary Clinton.