Donald Trump has denounced Monday’s FBI raid of his longtime personal lawyer’s New York office as a break-in, a “total witch hunt,” and even an attack on the United States.

But legal specialists say such raids, while unusual and aggressive, have to be vetted through an arduous legal process approved at the highest levels of the Department of Justice.

Prosecutors typically carry out such searches only if they fear the lawyer might tamper with evidence they are seeking, specialists said.

“It’s just a matter that you don’t trust them, that a subpoena won’t be enough to get what you need,” said Laurie Levenson, a professor at Loyola Law School in Los Angeles and a former prosecutor who was often appointed by courts to review documents seized in such raids.

Put more bluntly, prosecutors might have feared that attorney Michael Cohen would hide, or even destroy, incriminating evidence if they did not seize it first, said Martin Healy, chief counsel for the Massachusetts Bar Association.

“We don’t have the particulars on what they’re looking for,” Healy said. “We do know this is a very serious step when law enforcement raids an attorney’s office. It’s usually based on some very serious evidence that is likely to prove the attorney involved has done something not only unethical, but possibly illegal.”

Cohen’s lawyer has said the raid was carried out after a referral from Special Counsel Robert Mueller to federal prosecutors based in New York. Federal officials have not commented on the raid, and it is not clear whether it was related to Mueller’s investigation of alleged collusion between the Trump campaign and Russia.

Cohen has acknowledged paying pornographic film actress Stephanie Clifford to silence her about her relationship with Donald Trump. The FBI is investigating those payments, and agents seized e-mails, tax documents, and business records from Cohen’s office