The FBI has visited Michael Cohen's office and taken "communications" amid controversy over payments made to Stormy Daniels.
Mr Cohen has been at the centre of allegations made by Ms Daniels that she was paid to keep quiet after having sex with Donald Trump.
Stephen Ryan, who is Mr Cohen's lawyers, said certain "communications" had been seized.
He added: "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.
"The decision by the US attorney's office in New York to conduct their investigation using search warrants is completely inappropriate and unnecessary.
"It resulted in the unnecessary seizure of protected attorney-client communications between a lawyer and his clients."
A spokesman for Special Counsel Mueller was not able to comment.
Porn star Stormy Daniels seeks evidence from Donald Trump over payment
Mr Mueller is investigating claims Russia meddled in the 2016 US presidential election and possible collusion between Russia and the Trump campaign.
According to Mr Ryan, Mr Cohen has been co-operating with the authorities and has already handed over thousands of documents to investigators.
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The New York Times reported a source as claiming emails, tax documents and business records were seized on Monday.
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