The Justice Department, which is typically tasked with chasing major criminals, has found itself at the center of a case involving an ambitious tweeter. After noted security researcher Justin Shafer mentioned five Twitter users who regularly engage in vocal discussions about policy and law—@Popehat, @PogoWasRight, @dawg8u, @abtnatural, and @associatesmind—in a tweet with a single smiley-face emoji, the D.O.J. sent a subpoena to Twitter demanding personal information about all five users. Techdirt reports that the subpoena asks for details including the users’ names, addresses, IP addresses associated with their time on Twitter, phone numbers, and credit card or bank account numbers.

The subpoena is intended to address allegations that Shafer, who has a history of spotting weak encryption and drawing attention to it, cyberstalked an F.B.I. agent after the agency raided his home. In 2013, Shafer discovered that FairCom’s data-encryption package had actually exposed a dentist’s office to data theft. An F.T.C. settlement later validated Shafer’s reporting, but in 2016, when another dentist’s office responded to Shafer’s disclosure by claiming he’d violated the Computer Fraud and Abuse Act and broken the law, the F.B.I. raided his home and confiscated many of his electronics. Shafer was particularly annoyed at F.B.I. Special Agent Nathan Hopp, who helped to conduct the raid, and who was later involved in a different case: in March, he compiled a criminal complaint involving the F.B.I.’s arrest of a troll for tweeting a flashing GIF at journalist Kurt Eichenwald, who is epileptic. (The troll was charged with cyberstalking.) Shafer began to compile publicly available information about Hopp, sharing his findings on Twitter. The Twitter users named in the subpoena had started a separate discussion about Hopp, with one user calling Hopp the “least busy F.B.I. agent of all time,” a claim that prompted Shafer’s smiley-faced tweet.

Now the Justice Department says it believes Shafer was cyberstalking Hopp, and it‘s taking the unsettling step of collecting information about the other users Shafer engaged with on Twitter—a possible indication that the department believes they collaborated with him. Twitter, for its part, is fighting the subpoena, which legal experts say is unconstitutional. But the fact that it exists in the first place sets an alarming precedent, as it seems to imply that when it comes to publicly available information about law enforcement agents, Twitter users should keep their mouths shut.