(CNN) -- FBI agents raided a home of President Donald Trump's former campaign chairman Paul Manafort last month, a source familiar with the matter told CNN.
The agents seized materials in Manafort's home as part of the ongoing Russia investigation led by Justice Department Special Counsel Robert Mueller, the source said.
"FBI agents executed a search warrant at one of Mr. Manafort's residences. Mr. Manafort has consistently cooperated with law enforcement and other serious inquiries and did so on this occasion as well," Jason Maloni, a spokesman for Manafort, told CNN. He declined to provide further details.
The so-called no-knock warrant, which was first reported by The Washington Post, was served at Manafort's home in Washington's northern Virginia suburbs on July 26, the day after Manafort met with Senate intelligence committee investigators.
The tactic appears unusual for a case that has been under investigation for months and for which Manafort has already turned over hundreds of pages of documents to Senate investigators. The source told CNN the documents seized included financial and tax records and at least some of the information had already been provided to Senate investigators.
Since his appointment in May, Mueller has quietly gathered a team of more than three dozen attorneys, investigators and other staff in a nondescript office in Washington. Officials familiar with the probe describe it as akin to a small US attorney's office, with FBI agents and prosecutors assigned to separate groups looking into various aspects of the investigation.
These include groups of investigators and lawyers focused separately on Russian collusion and obstruction of justice, as well as the investigations focused on Manafort and former national security adviser Michael Flynn, a US official briefed on the investigation has told CNN.
So far, Trump's campaign has turned over approximately 20,000 pages to the Senate judiciary committee, which is investigating Russia's interference in the election, while Manafort turned over approximately 400 pages and Donald Trump Jr. turned over about 250 pages.
Fusion GPS, the firm that compiled a dossier at the center of the federal Russia probe, has not yet turned over any documents, according to the committee's spokesperson, though a source told CNN the firm plans to provide the committee with "thousands" of pages of documents Wednesday.
The spokesperson declined to provide details about the specific contents of the documents.
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