The raid, first reported by The New York Times, is an extraordinary development in a story that was already incredibly strange. Cohen wired $130,000 to adult actress Stormy Daniels in late 2016, during the waning days of the presidential campaign, in order to prevent her from speaking out about her alleged affair with Trump. A longtime employee of the Trump Organization and one of the president’s personal attorneys, Cohen has reportedly been under scrutiny by Special Counsel Robert Mueller, who is investigating the Russian effort to sway the 2016 election on Trump’s behalf, and whether the Trump campaign aided that effort. It’s unclear whether the raid is related to the special counsel’s investigation, or a separate inquiry being pursued by the U.S. Attorney’s Office in the Southern District of New York.

The raid was reportedly conducted after Mueller went to Deputy Attorney General Rod Rosenstein with evidence, and Rosenstein referred the matter to the U.S. attorney for the Southern District, Geoffrey Berman (Berman reportedly recused himself). According to the Times, federal agents seized “business records, emails, and documents related to several topics,” including Cohen’s payment to Daniels, and “privileged communications” between Cohen and his clients, according to Cohen’s attorney.

“You don’t just have to have the evidence that the documents may or may not exist, you have to show that there’s no other way to get them besides serving a warrant on the attorney, because of the sensitivity of attorney-client privilege,” David Gomez, a former FBI agent and a fellow at George Washington University’s Center for Cyber and Homeland Security, said.

On Tuesday, Trump tweeted “WITCH HUNT” and “Attorney–client privilege is dead!”

Some of the president’s supporters in the conservative press have been invoking attorney-client privilege, the legal rule that says communications between an attorney and a client are typically protected. But there are important exceptions.

“Records of conversations between Mr. Cohen and Mr. Trump are not necessarily privileged,” Bruce Green, a former federal prosecutor and a law professor at Fordham University, said. “If the conversations do not relate to a legal representation, but Mr. Cohen was providing business assistance or other non-legal services, the privilege probably will not apply.”

There is also something known as the crime-fraud exception to attorney-client privilege. “When the communications between an attorney and client are in furtherance of criminal activity, it’s viewed as an exception to attorney-client privilege,” Barrett said.

In cases where an attorney’s records are seized, a separate team of federal investigators, known as a “taint team,” will go through those records and sort out which are protected, and which prosecutors will be allowed to see or use. “There are various other limitations and exceptions that could make the privilege inapplicable. If it isn’t clear whether documents are privileged, the issue may get litigated,” Green said.