The FBI has conducted a pre-dawn raid on the Virginia home of Donald Trump's former campaign chairman Paul Manafort, his spokesman confirmed.
Federal agents are said to have seized files and other material on 26 July, a day after Mr Manafort voluntarily met the Senate Intelligence Committee.
Mr Manafort left the Trump campaign over questions about his foreign ties.
The FBI is leading one of several investigations into alleged Russian meddling in last year's US election.
Special counsel Robert Mueller, who heads up the FBI investigation, left Mr Manafort's Alexandria home with "various records", the Washington Post first reported.
A day before the raid, Mr Manafort voluntarily met a congressional panel that is also investigating Russia's alleged influence on the 2016 election.
Analysts say the search warrant suggests investigators wanted to be sure Mr Manafort would hand over all records in response to a grand jury subpoena.
Federal agents were said to be searching for tax documents and foreign bank records, US media reported.
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Mr Manafort's spokesman, Jason Maloni, confirmed that federal agents had "executed a search warrant at one of Mr Manafort's residences".
"Mr Manafort has consistently co-operated with law enforcement and other serious inquiries and did so on this occasion as well," he added.
The special counsel's office has declined to comment.