The FBI has raided the offices of Michael Cohen, the long-time personal lawyer for US President Donald Trump.
Law enforcement acted on a "referral" from Special Counsel Robert Mueller, who is investigating suspected Russian meddling in the 2016 election.
Officials in New York seized "privileged communications" between Mr Cohen and his clients, his lawyer said in a statement after the raid.
Documents regarding a payment to a porn actress were also seized, US media say.
Mr Cohen has come under increasing public scrutiny since he admitted to making a $130,000 (£92,000) payment to adult film star Stormy Daniels just days before the 2016 presidential election.
Ms Daniels - who claims that she had a sexual encounter with Mr Trump shortly after his wife Melania gave birth to their son - says she was paid to keep quiet about the affair.
"Today the US Attorney's Office for the Southern District of New York executed a series of search warrants and seized the privileged communications between my client, Michael Cohen, and his clients," Mr Cohen's lawyer Stephen M Ryan said in a statement.
"I have been advised by federal prosecutors that the New York action is, in part, a referral by the Office of Special Counsel, Robert Mueller."
Mr Ryan called the raid "inappropriate and unnecessary".
"It resulted in the unnecessary seizure of protected attorney client communications between a lawyer and his clients. These government tactics are also wrong because Mr Cohen has co-operated completely with all government entities, including providing thousands of non-privileged documents to the Congress and sitting for depositions under oath," the statement said.
Last week, Mr Trump said he was unaware of the payment to Ms Daniels.