Michael Cohen knows a lot of the president’s secrets. It’s an occupational hazard for the lawyer who acts as Donald Trump’s legal consigliere, first as the Trump Organization’s general counsel and then as his personal attorney since last year. Cohen has built a reputation not so much for his skills as an attorney, but for his sheer aggressiveness in defending his client.

That’s what made Monday’s FBI raid on his office, home, and hotel room so extraordinary. It appears to be the first search of its kind targeting a president’s personal lawyer in modern American history. Neither Richard Nixon’s attorneys during the Watergate crisis nor Bill Clinton’s legal counsel in the Starr investigations faced similar treatment from investigators. Cohen’s lengthy history with Trump and intimate knowledge means that the search could be a turning point in the Russia investigation.

Searching an attorney’s office is rare because the American legal system places great value on keeping sensitive discussions between clients and their lawyers confidential. “The attorney-client privilege is an exception to the general rule that the facts will come out,” Samuel Buell, a Duke University law professor, told me. “If we have evidence, we might nonetheless keep it out if it damages the ability of people to communicate with their lawyers.”

That’s why the Justice Department imposes additional checks when federal prosecutors target an attorney’s office. To obtain a search warrant, investigators generally have to persuade a judge that they have probable cause to believe they’ll find evidence of a crime. The U.S. attorney’s manual also outlines additional steps before an application can go to a judge. Prosecutors must first get approval from either the U.S. attorney for the jurisdiction or from the head of the Justice Department’s Criminal Division and consult with that division’s career lawyers.

“This is not your average, ordinary search warrant,” Stephen Vladeck, a University of Texas law professor, told me. “It suggests the government had quite a lot of reasons to think they were going to find some incriminating material.”

There are overlapping accounts of what investigators were searching for. The New York Times initially reported that the warrant covered documents related to Cohen’s $130,000 payment to Stephanie Clifford, known professionally as Stormy Daniels. The Washington Post reported on Monday night that federal investigators are scrutinizing Cohen for “possible bank fraud, wire fraud and campaign finance violations.” CNN said the search warrant also covered Cohen’s ownership of New York City taxi medallions as well as Karen McDougal’s deal with the National Enquirer that buried an account of her alleged relationship with Trump.

On Monday, the president responded to the search by furiously denouncing it to White House reporters as both a “break-in” and an “attack on our country.” Trump also floated the idea of firing Mueller, a maneuver he’s already attempted at least once before. On Tuesday morning, he proclaimed on Twitter:

American pop culture typically depicts attorney-client privilege as an impenetrable shield that protects both parties from repercussions. The reality is much less dramatic. Circumventing the privilege is “uncommon but by no means unprecedented, or even untoward,” Buell explained. “It’s just something for which there is a very high bar set so that it only happens infrequently and when absolutely necessary.”

The privilege itself also won’t apply to every communication between a lawyer and a client. A conversation between Trump, Cohen, and a third party might not be privileged, for example. Conversations that weren’t about substantive legal advice also wouldn’t qualify. Cohen could freely testify about those interactions; prosecutors would be able to use materials that document those conversations as evidence.

If Cohen challenges the use of privileged material in any future criminal cases, prosecutors would likely invoke what’s known as the crime-fraud exception. “The privilege does not extend to protect information even that would otherwise be covered that pertains to criminal or fraudulent activity,” Vladeck said. “If, for example—and this is wildly speculative—there are memorialized conversations where Trump tells Cohen to hire someone to intimidate Stormy Daniels, that wouldn’t be protected.”

That the search targeted the personal attorney for the president of the United States apparently prompted even greater scrutiny from the Justice Department’s upper ranks. The Times reported that Deputy Attorney General Rod Rosenstein, a Trump nominee, personally approved the raid on Cohen’s office, perhaps underscoring its potential significance.

Rosenstein serves as the acting attorney general for investigations related to the 2016 election after Attorney General Jeff Sessions recused himself last spring. In that role, he also oversees special counsel Robert Mueller’s wide-ranging probe into Russia’s election meddling and possible collusion with the Trump campaign. That investigation appears to have uncovered information that prompted the raid. Stephen Ryan, Cohen’s lawyer, told reporters that he had “been advised by federal prosecutors that the New York action is, in part, a referral” by Mueller.

Rosenstein may have determined that the information Mueller uncovered is beyond the special counsel’s mandate. That could be good news for the president, since it could indicate there’s some distance between whatever prompted the Cohen searches and Trump himself. On the other hand, it could also be a tactical move. Preet Bharara, who served as the U.S. attorney for Manhattan until his dismissal by Trump last year, floated the idea Tuesday night that investigators in his former office may be acting as a “clean team” for the special counsel.

“The idea is that you have a team go through it that’s not involved in the underlying investigation or prosecution,” Vladeck said. “You basically create a wall between the folks who look at all the material and make sure that there’s nothing privileged, and the folks who get access to the properly segregated and culled take from the searches.”

Using a clean team avoids epistemological problems that could undermine an investigation’s integrity if prosecutors get access to a lawyer’s files. “Once a prosecutor has been exposed to that material, they can’t unlearn it,” Buell said. “It could be that Mueller didn’t have the ability to create a clean team within his own operation. He’s not going to hire a bunch of new staff to conduct a search, so he may have decided to use a U.S. attorney’s office instead.”

Mueller’s opacity makes it hard to tell if that’s what’s happening here. “It’s just as possible that Rosenstein concluded that the search was sufficiently tangential to Mueller’s investigation that it made sense to hand it over to somebody else,” Vladeck said. “But either way, it has the effect of making this look, at least outwardly, less political, even if there are folks who won’t believe that.”

Even if it’s a completely distinct investigation, it’d be hard to investigate Cohen without also taking a look at some of the president’s legal affairs. If the Manhattan office finds evidence that’s relevant to Mueller’s probe, it would be up to Rosenstein to regulate the flow of information between the two groups.

This makes Cohen’s legal troubles a potential liability for the president, no matter the offense involved. Mueller managed to secure a plea deal from former National Security Advisor Michael Flynn last year by pressing him on a tangential criminal matter; he’s currently trying to do the same against former Trump campaign chairman Paul Manafort. Attorney-client privilege limits how much Cohen could say if prosecutors decide to use similar tactics against him, but its protections are far from absolute.

“The privilege is designed to help [attorney-client] relationships function well because it’s to society’s advantage,” Buell said. “It’s not to society’s advantage to cover up crimes.”