Social media giant Reddit has been ordered to appear before a grand jury in Washington, D.C., as part of a federal effort to unmask anonymous online critics of the Trump administration’s immigration crackdown.

According to a subpoena obtained by The Intercept, Reddit has until April 14 to provide a wide range of personal data on one of its users, whom U.S. Immigration and Customs Enforcement agents have been trying unsuccessfully to identify for more than a month.

Attorneys for the Reddit user say their client’s posts and their anonymity are squarely protected under the First Amendment and that ICE’s use of a grand jury marks a disturbing escalation for the agency after seeing its previous efforts to investigate political speech quashed in court. The subpoena was issued by federal prosecutors in the capital after ICE’s effort to identify the same user failed in a Northern California federal court. (The U.S. attorney’s office in Washington declined to comment on the case.)