The dramatic F.B.I. raid executed Monday against Michael Cohen, the president’s personal attorney, marks a turning point in the Justice Department’s investigations into Donald Trump and his associates. As Trump noted Tuesday morning, in his own madcap way, attorney-client privilege is a cornerstone of the U.S. legal system. For federal agents to have searched Cohen’s office and hotel room, prosecutors must have convinced a judge that a raid was necessary to obtain evidence of a crime that Cohen might have attempted to hide or destroy if he was issued a subpoena. The implications, for Cohen and for Trump, are potentially explosive. “I suppose you could say that it is Trump’s moment of truth,” Bob Bauer, the former Obama White House general counsel, told me. “It is the red line he always seemed to draw in his bitter complaints about the Mueller investigation—now here we are. Cohen would not be a subject or target that Trump would wish to have flipped against him.”

The Justice Department’s interest in Cohen appears to be connected to his efforts to silence two women, in the run-up to the 2016 election, who claimed to have had affairs with Trump. As The New York Times first reported, and several other publications quickly confirmed, federal agents were seeking records related to former Playboy model Karen McDougal—who was paid $150,000 by Trump ally David Pecker’s American Media Inc., which owns The National Enquirer, burying her story—and Stormy Daniels, who Cohen paid $130,000 to sign a non-disclosure agreement. Any payment facilitated on behalf of the Trump campaign could be considered a violation of campaign finance laws, which then set the individual contribution limit at $2,700. (Stephen Ryan, an attorney for Cohen, characterized the raid as “completely inappropriate and unnecessary,” and said it had “resulted in the unnecessary seizure of protected attorney-client communications between a lawyer and his clients.”)

Legal experts agree this is a major development. “It’s an enormously aggressive move for any prosecutor to make, particularly given Cohen’s client,” said Sol Wisenberg, a deputy special prosecutor during the Starr investigation. Neal Katyal, who served as acting solicitor general under Obama, described it as “very bad news” for Cohen. “D.O.J. only raids once they have concluded that lesser options—like cooperation—aren’t working and that there is a risk that evidence may be destroyed,” he said. The barrier to convincing a judge to approve a search warrant would be even higher in a case involving a sitting president. More significant, a judge would require evidence of potential illegal behavior between Trump and Cohen before approving the seizure of any communications involving both of them. “If the warrant specifically authorized the seizure of attorney-client communications, the government would have had to convince a federal magistrate that the crime-fraud exception was in play,” Wisenberg added.

Andrew Hall, who represented former Nixon adviser John Ehrlichman during the Watergate scandal, offered a succinct summary of the distinction: “If a client goes to a lawyer and confesses, ‘I committed a crime, I stole money, I shot somebody, I killed somebody’—whatever the crime is, it doesn’t matter. That confession is within the attorney-client privilege. It cannot be discovered,” he said. “However, if the client is attempting to use the lawyer, or the lawyer is volunteering to be a part of an ongoing crime, there is no attorney-client privilege.”

Even then, prosecutors have to be extraordinarily careful about what communications remain privileged. “Going in and seizing a lawyer’s computer [poses] a lot of danger to the government because there is, built into that process, the invasion of the attorney-client privilege,” Hall continued. Renato Mariotti, a former federal prosecutor in Illinois, elucidated the difficulty of differentiating between the two. “When you obtain a search warrant, you list in the search warrant the items to be seized,” he explained. “You offer reasons or evidence to support why each one of those things will be found in the location, and why those things are relevant to an ongoing investigation,” which means investigators would have had to outline why communications between Trump and Cohen were pertinent to the investigation and possibly did not fall under attorney-client privilege. Next, a “taint team” will have to determine what communications between the president and his attorney might fall into this category. “If they are discussing how to commit a crime, that is also not privileged,” he said. “If I was Trump, that would be my biggest concern.”