Twitter — owner of the president’s favorite communications platform — is fighting the U.S. Department of Homeland Security and other federal officials who it alleges in a suit today are trying to “unmask” a user who has been critical of the Donald Trump administration.

“The rights of free speech afforded Twitter’s users and Twitter itself under the First Amendment of the U.S. Constitution include a right to disseminate such anonymous or pseudonymous political speech,” Twitter told the U.S. District Court in Northern California.

The government, it adds, has “not come close” to demonstrating that there’s been a criminal or civil offense and that “unmasking the user’s identity is the least restrictive means for investigating that offense.”

The suit says that on March 14 the government sent Twitter an administrative summons demanding it provide “records that would unmask, or likely lead to unmasking, the identity of the person(s)” behind an account named @ALT_USCIS that criticized DHS’ Citizenship and Immigration Services unit.

This is one of several accounts created by people who claim to be current or former employees of federal agencies and “provide views and commentary that is often vigorously opposed, resistant, or ‘alternative’ to the official actions and policies of the new Administration,” the suit says. Other accounts with similar @ALT titles are critical of the Labor Department and the Bureau of Land Management.

Disclosure of the anonymous tweeters “would have a grave chilling effect on the speech of that account in particular and on the many other ‘alternative agency’ accounts that have been created to voice dissent to government policies,” Twitter says.

The @ALT_USCIS account explicitly says it does not represent “the views of DHS or USCIS.”

Twitter says that on March 14 the government faxed a summons ordering the company to produce records disclosing “User names, account login, phone numbers, mailing addresses, and I.P. addresses” for the account on March 13 — the day before the summons was sent.

The order said it was “in connection with an investigation or inquiry to ascertain the correctness of entries, to determine the liability for duties, taxes, fines, penalties, or forfeitures, and/or to ensure compliance with the laws or regulations.”

When Twitter contacted a Customs and Border Protection agent who sent the summons, the suit says he “stated vaguely that he is conducting an investigation. But he did not identify any law or laws that he believed had been broken or point to any evidence substantiating any such belief—such as particular Tweets that he believes were unlawful.”

This week the company told the account holder about the summons, and told the government that it would challenge the order.