FILE PHOTO: Manafort of Republican presidential nominee Trump's staff listens during a round table discussion on security at Trump Tower in the Manhattan borough of New York Thomson Reuters

The FBI conducted a predawn July raid on the home of President Donald Trump's campaign chairman, Paul Manafort, as part of its ongoing investigation into possible collusion between the Trump campaign and Russia, The Washington Post reported Wednesday.

FBI agents working with Robert Mueller, who was appointed special counsel to lead the investigation after James Comey was fired as FBI director in May, left Manafort's home in Washington's northern Virginia suburbs "with various records," according to The Post.

The New York Times reported shortly afterward that investigators were looking for tax documents and foreign banking records, documents "typically sought when investigating violations of Bank Secrecy Act," Times reporter Adam Goldman noted.

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 only reason to do a search warrant on a target who is ostensibly cooperating with the investigation is a lack of trust," said Kenneth Julian, a partner at Manatt, Phelps & Phillips who served for more than 11 years as a federal prosecutor in the Central and Eastern districts of California.

Manafort's spokesman, Jason Maloni, confirmed in a statement that "FBI agents executed a search warrant at one of Mr. Manafort's residences" and said Manafort had "cooperated."

Donald Trump strategist Paul J. Manafort, left, chats with former presidential candidate Ben Carson as they head to a Trump for president reception at the Republican National Committee Spring Meeting, Thursday, April 21, 2016, in Hollywood, Fla. AP Photo/Wilfredo Lee

The raid happened one day after Manafort met privately with the Senate Intelligence Committee and provided notes about his meeting with two Russian lobbyists at Trump Tower in June 2016. Trump's son Donald Jr. and son-in-law, Jared Kushner, also attended the meeting, which was not disclosed by the campaign at the time.

An adviser close to the White House told The Post that the documents obtained by the FBI included material Manafort had already given to congressional investigators.

But Jack Sharman, a white-collar lawyer in Birmingham, Alabama, who was appointed special counsel to the House Banking Committee for the Whitewater investigation of President Bill Clinton, said a search warrant like this "is designed to send a message."'

"One purpose of such a raid is to bring home to the target the fact that the federal prosecution team is moving forward and is not going to defer to or rely on Congress," added Sharman, who also recently served as special counsel to the Alabama House Judiciary Committee for the impeachment of Gov. Robert Bentley.

'They don't believe he is fully cooperating'

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," Miller said.

"The fact they cut any cooperation short and raided his house suggests they don't believe he is fully cooperating and that there are documents or electronic files, possibly contained on computers at his house, in his possession that they did not trust him to turn over."

A former federal prosecutor, Renato Mariotti, explained on Twitter that to obtain a search warrant, the FBI must work with a federal prosecutor to lay out evidence in an affidavit showing probable cause that a search of the suspect's home will uncover evidence of a crime.

"Why would the FBI want to search the home of a subject like Manafort? Because there may be documents, ledgers, and other records," Mariotti said. "More importantly, there may be computers and other digital media that contain communications."

Paul Manafort in Cleveland on July 14, 2016. Reuters

Also chiming in on Twitter was Asha Rangappa, a former FBI special agent who noted that, in obtaining a warrant, the FBI evidently convinced a judge that there was probable cause a crime had been committed — and "that there was some risk that Manafort may try to remove conceal evidence despite cooperation."

"In order to get a search warrant, FBI agents had to swear to their belief that fruits of a crime would be found in Manafort's home," said Julian, the former California-based federal prosecutor.

Rangappa noted that when she was at the FBI, raids were often conducted at 5 a.m. "to catch target unawares, so they cannot destroy or remove evidence." She added that anything the government finds "can be used to leverage Manafort," especially if it shows "that he had been lying" to the government or Congress.

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.

The ledger, and Manafort's activities in Ukraine more broadly, were examined more closely following Yanukovych's ouster on corruption charges in 2014. Manafort has been associated with at least 15 bank accounts and 10 companies in Cyprus, dating back to 2007, NBC reported in March.

On March 22, the Associated Press reported that Manafort was paid $10 million from 2006 to 2009 to lobby on behalf of Russian billionaire Oleg Deripaska, a close ally of Russian President Vladimir Putin, using a strategic "model" that the AP said Manafort wrote would "greatly benefit the Putin Government if employed at the correct levels with the appropriate commitment to success."

Manafort has insisted that he has never received any illicit cash payments. But he has a "pattern" of using shell companies to purchase homes "in all-cash deals," as WNYC has reported, and then transferring those properties into his own name for no money and taking out large mortgages against them.

Manafort's tendency to form shell companies to purchase real estate is not illegal. But it has raised questions about how much Manafort has been paid throughout the decades he's spent as a political consultant, and by whom.