The very big news of the day: FBI agents raided the law office of Michael Cohen, President Donald Trump's lawyer who was involved in payment of $130,000 to adult performer "Stormy Daniels" for a nondisclosure agreement. Some reports suggest they also raided his home.

Recently I've been listening to the podcast Slow Burn, about Watergate. There's a fascinating theme throughout it: When you're living a historical event, how do you know? How can you tell when a development is a big deal?

This is a big deal. It's very early on, but here's some things we can already tell.

1. According to Cohen's own lawyer, the U.S. Attorney's Office for the Southern District of New York (widely regarded within itself as being the most important and prestigious U.S. Attorney's Office in the country) secured the search warrants for the FBI, based on a referral from Robert Mueller's office. Assuming this report is correct, that means that a very mainstream U.S. Attorney's Office—not just Special Counsel Robert Mueller's office—thought that there was enough for a search warrant here.

2. Moreover, it's not just that the office thought that there was enough for a search warrant. They thought there was enough for a search warrant of an attorney's office for that attorney's client communications. That's a very fraught and extraordinary move that requires multiple levels of authorization within the Department of Justice. The U.S. Attorney's Manual (USAM)—at Section 9-13.320—contains the relevant policies and procedures. The highlights:

The feds are only supposed to raid a law firm if less intrusive measures won't work. As the USAM puts it:

In order to avoid impinging on valid attorney-client relationships, prosecutors are expected to take the least intrusive approach consistent with vigorous and effective law enforcement when evidence is sought from an attorney actively engaged in the practice of law. Consideration should be given to obtaining information from other sources or through the use of a subpoena, unless such efforts could compromise the criminal investigation or prosecution, or could result in the obstruction or destruction of evidence, or would otherwise be ineffective.

Such a search requires high-level approval. The USAM requires such a search warrant to be approved by the U.S. attorney—the head of the office, a presidential appointee—and requires "consultation" with the Criminal Division of the U.S. Department of Justice. This is not a couple of rogue AUSAs sneaking in a warrant.

Such a search requires an elaborate review process. The basic rule is that the government may not deliberately seize, or review, attorney-client communications. The USAM—and relevant caselaw—therefore require the feds to set up a review process. That process might involve a judge reviewing the materials to separate out what is privileged (or what might fall within an exception to the privilege), or else set up a "dirty team" that does the review but is insulated from the "clean team" running the investigation. Another option is a "special master," an experienced and qualified third-party attorney to do the review. Sometimes the reviewing team will only be identifying and protecting privileged material. Sometimes the reviewing team will be preparing to seek, or to implement, a court ruling that the documents are not privileged. (Robert Mueller is aggressive on this sort of thing; he already sought and obtained a court ruling that some of Paul Manafort's communications with his lawyers were not privileged because they were undertaken for the purpose of fraud—the so-called "crime-fraud exception" to the attorney-client privilege.)

3. A magistrate judge signed off on this. Federal magistrate judges (appointed by local district judges, not by the president) review search warrant applications. A magistrate judge therefore reviewed this application and found probable cause—that is, probable cause to believe that the subject premises (Cohen's office) contains specified evidence of a specified federal crime. Now, magistrate judges sometimes are a little too rubber-stampy for my taste (notably, recall the time that a magistrate judge signed off on a truly ludicrous gag order forbidding Reason from revealing that it had been served with a subpoena for information identifying commenters). But here, where the magistrate judge knew that this would become one of the most scrutinized search warrant applications ever, and because the nature of the warrant of an attorney's office is unusual, you can expect that the magistrate judge felt pretty confident that there was enough there.

4. The search warrant application (the lengthy narrative from the FBI agent setting for the evidence) is almost certainly still under seal, and even Michael Cohen doesn't get to see it (yet). But the FBI would have left the warrant itself—and that shows (1) the federal criminal statutes they were investigating, and (2) the list of items they wanted to seize. Much can be learned for those. Assuming Michael Cohen doesn't release it, watch for it to be leaked.

Again: This is a big deal.

It's early times. Watch for the search warrant itself—that will show us what crimes they are investigating and what documents they think are probative of that crime. Watch also for what Michael Cohen's lawyers do in the struggle to compel arbitration with Stormy Daniels in a federal court in Los Angeles—the search warrant dramatically complicates whether Cohen can, or should, submit to any questions in that case. Be skeptical of the surge of misinformation and inaccurate legal takes that are certain to drop. But watch. This is historic.

(A version of this story originally appeared at the Popehat blog)