Twitter has filed a lawsuit against the U.S. Department of Homeland Security, asking a federal court to protect its right to prevent the government from unmasking users who have spoken critically of President Donald Trump.

In a lawsuit filed today, Twitter said Homeland Security in March demanded that Twitter release information about a user associated with one of the accounts purportedly run by dissenting employees working for various federal government agencies.

According to court documents reviewed by Adweek, the Department of Homeland Security on March 14 sent a summons to Twitter requesting information about @Alt_uscis, one of several “rogue” Twitter accounts that surfaced in the days following Trump’s inauguration and has since gained tens of thousands of followers. The summons demanded the company provide certain records about the user, including their real name, phone number, mailing address and IP address. The letter also threatened legal action against Twitter if the company didn’t comply.

According to Twitter, Homeland Security is “unlawfully abusing” a “limited-purpose investigatory tool” in order to unmask the user, which the company asserted is a violation of both the user’s and Twitter’s right to free speech. A copy of the summons included with the lawsuit said the user’s information was needed in order to assure they were in compliance with federal tax guidelines.

“Compelled disclosure of the identities of Twitter users who have engaged in pseudonymous speech would chill their exercise of the constitutionally protected right to speak anonymously,” according to Twitter’s court documents. “When rights of free speech—especially anonymous free speech—are at stake, the courts generally permit an organization or business to assert those rights on behalf of its members or customers.”

Twitter added, “A time-honored tradition of pseudonymous free speech on matters of public moment runs deep in the political life of America.”

The lawsuit, which described the “alternative agency” accounts as an “innovative class of American speakers,” also mentions other accounts such as those purportedly run by employees of agencies including the Department of Labor and the Environmental Protection Agency.

According to Twitter, the DHS agent asked Twitter not to notify the user the government was trying to unmask them. But on Tuesday, Twitter notified the user about the summons while also notifying the agent of the company’s plans to take legal action. In a tweet this afternoon, @Alt_uscis tweeted a quote from the U.S. Constitution that cites the protection of free speech.

According to court documents, DHS has still not notified the company of any intent to withdraw the summons.

A spokesperson for the department declined to comment on the lawsuit, citing a policy related to pending litigation.