Twitter has launched a lawsuit against the Trump administration, after the Department of Homeland Security sought to unmask an anonymous user of an anti-Trump account.

According to Twitter’s suit, filed today in Northern California District Court, US Customs and Border Protection has attempted to use a “limited-purpose investigatory tool” to unmask the owner of the Twitter account “@ALT_uscis.” The account, one of several “alt” or “rogue” government accounts that appeared in the wake of Trump’s ascent to the presidency, was used “to express public criticism of the Department and the current Administration,” according to Twitter’s complaint.

In the suit, Twitter writes that @ALT_uscis has purported to be a dissenting member of US Citizenship and Immigration Services. On March 14th, Twitter received a summons from Customs requesting records that could reveal the identity of the account’s operator, including IP logs and any associated phone number or mailing address. Twitter writes that it later informed the account holder of the summons, and told the agency of its intent to fight the order in court.

“a grave chilling effect on the speech of that account”

In addition to the Department of Homeland Security and its subagency, the lawsuit names four individuals as defendants: DHS secretary John Kelly, acting CBP commissioner Kevin McAleenan, and special agents Stephen P. Caruso and Adam Hoffman, who issued and served the order itself.

Twitter’s lawsuit focuses on whether Customs has the legal authority to make such a request. CBP’s initial order invoked a statute largely related to taxes on the importation of merchandise into the US, which Twitter argues is insufficient authority to unmask a user. Twitter also writes that unmasking the account “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.” Unmasking the account, the company contends, would infringe on political speech protected by Supreme Court rulings. The ACLU has pledged to support Twitter in the case.

Twitter declined to comment on the suit. Shortly after news of the lawsuit broke, the account @ALT_uscis responded with the following tweet.

@ALT_uscis is one of a string of accounts claiming to be operated by rogue government employees, formed in the wake of Donald Trump’s inauguration. It’s unclear whether any of the accounts are authentic — including @ALT_uscis — although many purport to offer an insider perspective. The Trump administration has responded aggressively to leaks in the past, including on-the-spot phone checks of White House staffers suspected of sharing information with the press.

The order served to Twitter is embedded below.