FBI agents raided the office of President Trump's longtime personal lawyer Michael Cohen on Monday at the request of special counsel Robert Mueller, an attorney for Cohen said.

The New York Times reported that agents collected documents related to Cohen's October 2016 payment of $130,000 to porn star Stormy Daniels, who says she had an affair with Trump.

Cohen claims he paid Daniels without Trump's knowledge, which experts say would amount to professional misconduct and possibly an illegal campaign contribution.

Trump — long frustrated by Mueller's probe, which began looking for evidence of collusion between his campaign and Russia — was watching coverage of the events on TV, a White House official told multiple outlets Monday afternoon.

Lawyer Stephen Ryan, who is representing Cohen, called the raid "completely inappropriate and unnecessary" in a statement.

“Today the U.S. 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,” Ryan said. “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.”

Ryan said that "[t]hese government tactics are also wrong because Mr. Cohen has cooperated completely with all government entities, including providing thousands of non-privileged documents to the Congress and sitting for depositions under oath."

Peter Carr, a spokesman for Mueller, would not discuss the raid. "We will decline to comment," he said in an email.

A source told CBS News that agents also raided Cohen's residence, which like his office is in New York. The law firm Squire Patton Boggs, which provided Cohen's office space, said in a statement that its “arrangement with Mr. Cohen reached its conclusion, mutually and in accordance with the terms of the agreement."

Michael Avenatti, an attorney representing Daniels, whose real name is Stephanie Clifford, in challenging the 2016 non-disclosure deal brokered by Cohen, said he wasn't surprised by the development, which he said could spell trouble for Trump.

"As I predicted last week on CNN and MSNBC, Mr. Cohen has been placed in the crosshairs by Mr. Trump," Avenatti said. "He has been set-up to take the fall. An enormous amount of misplaced faith has been placed on his shoulders and I do not believe he has the mettle to withstand it. If I am correct, this could end very, very badly for Mr. Trump and others."