Story highlights The agents seized materials in Manafort's home as part of the ongoing Russia investigation

The tactic appears unusual for a case that has been under investigation for months

Washington (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.

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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.