Although it is unusual for the Justice Department to seek a lawyer’s materials, the United States attorneys’ manual contains procedures for how to get both subpoenas and search warrants in such cases. It tells investigators to exhaust all other ways of obtaining evidence first “to avoid impinging on valid attorney-client relationships.” It also encourages them to use a subpoena if possible. But it also acknowledges that sometimes a search warrant may be justified, even though it is more intrusive, such as if there is reason to believe the recipient would destroy the evidence rather than turn it over.

What is lawyer-client privilege?

Most of the time, if someone possesses information that is relevant to getting at the truth about a suspected crime, the rules require that person to disclose such evidence, and prosecutors can use those facts in court. But courts have created exceptions to protect confidential communications between a client and a lawyer and a lawyer’s work product on behalf of a client, like notes and files the lawyer gathered in anticipation of litigation.

The idea, said Samuel W. Buell, a former federal prosecutor who teaches white-collar criminal law at Duke University, is that it is generally in the broader public interest to encourage people to consult lawyers and talk candidly with them. That way, lawyers can steer their clients toward lawful conduct in the first place, and better-informed lawyers will make the legal system function more smoothly.

Against that backdrop, Mr. Buell said, the search warrant for Mr. Cohen’s office was “not going to happen unless they feel that they have to, and they think they have something that is really serious going on.”

“I would say that for any law-office search warrant,” he continued, “but we just happen to be talking about a lawyer for the president of the United States.”