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FBI agents raided the Alexandria, Va., home of President Donald Trump’s former campaign chairman late last month, using a search warrant to seize documents and other materials, according to people familiar with the special counsel investigation into Russian meddling in the 2016 election.

Federal agents appeared at Paul Manafort’s home without warning in the predawn hours of July 26, the day after he met voluntarily with the staff for the Senate Intelligence Committee.

It provides the clearest evidence yet that special counsel Robert Mueller is aggressively pursuing the investigation into the campaign’s dealings with Russia and that his inquiry has broadened to include questions about Manafort’s finances.

The search warrant was wide-ranging and FBI agents working with Mueller departed with various records. Jason Maloni, a spokesman for Manafort, confirmed that agents executed a warrant and that Manafort cooperated with the search.

Manafort has been voluntarily producing documents to congressional committees investigating Russia’s interference in the 2016 presidential election. The search indicated that investigators may have argued to a federal judge that they had reason to believe Manafort could not be trusted to turn over all records in response to a grand jury subpoena. Unlike subpoenas, which a grand jury can issue, a warrant requires prosecutors to persuade a judge that probable cause exists to believe that a crime may have been committed.

It could also have been intended to send a message to Trump’s former campaign chairman that he should not expect gentle treatment or legal courtesies from Mueller’s team.

The documents included materials that Manafort had already provided to Congress, said people familiar with the search. “If the FBI wanted the documents, they could just ask [Manafort] and he would have turned them over,” said one adviser close to the White House.

Josh Stueve, spokesman for Mueller, declined to comment, as did Reginald Brown, an attorney for Manafort.

“Mr. Manafort has consistently cooperated with law enforcement and other serious inquiries and did so on this occasion as well,” Maloni said.

Mueller, who is working with a grand jury, has increased legal pressure on Manafort, consolidating under his authority a series of unrelated investigations into various aspects of Manafort’s professional and personal life.

Manafort’s allies fear that Mueller hopes to build a case against Manafort unrelated to the 2016 campaign, in hopes that the former campaign operative would provide information against others in Trump’s inner circle in exchange for lessening his own legal exposure.

The significance of the records seized is unclear. Manafort has provided documents to congressional committees, which are said to include notes that Manafort took during a meeting with Donald Trump Jr. and a Russian lawyer at Trump Tower in June 2016.

E-mails show that Trump Jr. took the meeting and invited Manafort after he was promised damaging information about Hillary Clinton as part of a Russian government effort to assist his father’s campaign.

The Tribune Washington bureau contributed to this report.