FBI agents raided the Alexandria, Va., home of President Trump’s former campaign chairman, arriving in the early-morning hours last month and seizing documents and other materials related to the special counsel investigation of Russian meddling in the 2016 election.

The raid, which occurred without warning on July 26, signaled an aggressive new approach by special counsel Robert S. Mueller III and his team in dealing with a key figure in the Russia inquiry. Manafort has been under increasing pressure as the Mueller team looked into his personal finances and his professional career as a highly paid foreign political consultant.

Using a search warrant, agents appeared the day Manafort was scheduled to testify before the Senate Judiciary Committee and a day after he met voluntarily with Senate Intelligence Committee staff members.

The search warrant requested documents related to tax, banking and other matters. People familiar with the search said agents departed the Manafort residence with a trove of material, including binders prepared ahead of Manafort’s congressional testimony.

Investigators in the Russia inquiry have previously sought documents with subpoenas, which are less intrusive and confrontational than a search warrant. With a warrant, agents can inspect a physical location and seize any useful information. To get a judge to sign off on a search warrant, prosecutors must show that there is probable cause that a crime has been committed.

“I think it adds a shock and awe enforcement component to what until now has followed a natural path for a white-collar investigation,” said Jacob Frenkel, a former federal prosecutor. “More so than anything else we’ve seen so far, it really does send a powerful law enforcement message when the search warrant is used. . . . That message is that the special counsel team will use all criminal investigative tools available to advance the investigation as quickly and as comprehensively as possible.”

Jason Maloni, a spokesman for Manafort, confirmed that agents executed a warrant at one of the political consultant’s homes and that Manafort cooperated with the search.

Manafort has been voluntarily providing documents to congressional committees investigating Russia’s election interference. The search warrant indicates that investigators may have argued to a federal judge that they had reason to think Manafort could not be trusted to turn over all records in response to a grand jury subpoena.

The raid also could 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 members, who already have begun combing through Manafort’s complicated financial past.

The documents seized in the raid include materials Manafort already had provided to Congress, said people familiar with the search, and the significance of what was obtained remained unclear Wednesday evening.

“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,” said Maloni, the Manafort spokesman.

Sen. Richard Blumenthal (D-Conn.), a member of the Judiciary Committee and a former U.S. attorney, called the search “a significant and even stunning development,” noting that such raids are generally reserved for “the most serious criminal investigations dealing with uncooperative or untrusted potential targets.”

“A federal judge signing this warrant would demand persuasive evidence of probable cause that a serious crime has been committed and that less intrusive and dramatic investigative means would be ineffective,” he said in a statement.

Mueller has increased legal pressure on Manafort, consolidating under his authority unrelated investigations of 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 the hope that he would provide information against others in Trump’s inner circle in exchange for lessening his legal exposure.

Manafort has provided more than 300 pages of documents to the Senate Judiciary Committee and the Senate and House intelligence committees. The information includes notes Manafort took while attending a meeting with Donald Trump Jr. and a Russian lawyer at Trump Tower in June 2016.

Manafort turns over notes from Trump Tower meeting with Russian lawyer

Emails show that Trump Jr. accepted the meeting and invited Manafort after he was told that the Russian lawyer would provide damaging information about Democrat Hillary Clinton as part of a Russian government effort to assist his father’s campaign.

The special counsel’s office has subpoenaed records and testimony related to the Trump Tower meeting, people familiar with the investigation said.

Last week, the Trump campaign turned over more than 20,000 pages of documents to congressional investigators examining Russian efforts to influence the election. Manafort and Trump Jr. separately turned over hundreds of documents requested by congressional committees.

Manafort had been subpoenaed to appear before the Senate Judiciary Committee on July 26, the day of the raid. Ultimately, however, the subpoena was withdrawn, and he did not testify. The committee chairman, Sen. Charles E. Grassley (R-Iowa), and Sen. Dianne Feinstein (Calif.), the panel’s top Democrat, said in a joint statement that they had dropped the demand after Manafort began producing documents.

Manafort, an experienced political operative who was once a business partner of Trump confidant Roger Stone, was hired to professionalize the Trump campaign at the recommendation of another Trump friend, Tom Barrack, an international real estate investor.

As a political consultant, Manafort traveled the world, at times offering advice to despots and dictators.

His decade of work in Ukraine on behalf of a Russia-friendly political party has drawn attention from the FBI. In Kiev, he advised the Party of Regions, helping to elect former president Viktor Yanukovych, who was ousted from power amid public protests in 2014 and fled to Russia.

A few days after attending the Trump Tower meeting in June 2016 at the urging of Trump Jr., Manafort was named chairman of Trump’s campaign, following the ouster of campaign manager Corey Lewandowski. Manafort remained in that role through the Republican National Convention and into August, when he resigned amid growing scrutiny of his work in Ukraine. The New York Times reported at the time that his name was found in a ledger of off-the-books cash payments made by the Party of Regions. Manafort has denied receiving any improper payments from the party.

Still, the work was lucrative. In June, Manafort filed Foreign Agents Registration Act paperwork with the Justice Department providing new details about his work in Ukraine. The documents showed that his firm was paid $17.1 million for its work in Ukraine over two years before expenses.

Manafort’s retroactive filing about his Ukraine work came as investigators scrutinized his foreign lobbying and began to aggressively probe his financial past.

During the spring, prosecutors issued subpoenas seeking information about his income sources and financial transactions. One subpoena reviewed by The Washington Post sought information “concerning contracts for work . . . communication or other records of correspondence” related to about two dozen people and businesses that appeared to be connected to Manafort or his wife, including some who worked for Manafort while he was a political consultant in Ukraine.

The subpoena was issued by a federal grand jury in the Eastern District of Virginia, where Manafort’s consulting business has been located. Also named in the subpoena were various corporations formed in Cyprus. Manafort appeared to have routed money he earned in Ukraine and in business dealings with a Russian business magnate through Cyprus-based bank accounts.

After Mueller was appointed, his team took over the Manafort investigation that was underway in Virginia.

State authorities in New York also have issued subpoenas seeking information about Manafort’s real estate loans.

And Mueller’s office has taken over an investigation of financial dealings that involved Manafort and his son-in-law, a real estate investor, including real estate transactions in California.

An earlier version of this report incorrectly described an early-morning FBI raid on the Alexandria home of former Trump campaign chairman Paul Manafort as occurring "in the pre-dawn hours." This report has been updated.

Philip Rucker, Matt Zapotosky and Julie Tate contributed to this report.