Washington (Reuters): A raid on the Virginia home of President Donald Trump 's former 2016 election campaign manager showed an investigation of possible ties between the campaign and Russia is intensifying and focused on the financial dealings of Trump associates, sources familiar with the probe said.

Longtime political consultant and lobbyist Paul Manafort is being investigated for possible money laundering and has been targeted as someone who might testify against former colleagues, said two people familiar with the work of Special Counsel Robert Mueller.

Manafort's spokesman Jason Maloni confirmed on Wednesday that the FBI executed a search warrant at one of Manafort's homes.

"Mr. Manafort has consistently cooperated with law enforcement and other serious inquiries and did so on this occasion as well," Maloni said in an email.

Manafort's house in Alexandria, near Washington, was raided in the early morning of July 26, the Washington Post reported. The previous day Manafort had met with Senate Intelligence Committee staff, the Washington Post reported, citing unidentified people familiar with the probe .

Tax documents and financial records were sought by agents for Mueller in the raid , the New York Times reported, citing a person familiar with the matter.

Mueller's team is poring over Manafort's financial and real estate records in New York and his involvement in Ukrainian politics, the two sources told Reuters last month.

Democratic Senator Richard Blumenthal called the raid a "highly significant step" and said it was "typical of the most serious criminal investigations dealing with uncooperative or untrusted potential targets."

The FBI did not immediately return a request for comment. Joshua Stueve, a spokesman for Mueller's office, declined to confirm the raid .

Trump has been attacking Blumenthal on Twitter this week after the senator urged Mueller to press his inquiry forward.

US intelligence agencies have concluded that Russia interfered in the presidential race, in part by hacking and releasing emails embarrassing to Trump 's opponent, Democrat Hillary Clinton, to help him get elected in November.

Trump has called Mueller's investigation a "witch hunt." Allegations of collusion between Trump associates and Moscow have hounded the Republican president since he took office in January, presenting a major distraction from his policy agenda.

Russia has repeatedly denied meddling in the US election.