has sued the Trump administration over attempts of the Department of Homeland Security (DHS) to unmask the identity of users behind an account critical of the government. The account in question, @ALT_USCIS, is one of several alt-government accounts which were launched after the new administration took office and proceeded to silence some official Twitter government accounts.

The lawsuit, filed in the Northern District of California, alleges that the DHS tried to use a “limited-purpose investigatory tool” to find out who is behind the account in question, which has been critical of the immigration policy of the current administration. “The rights of free speech afforded to Twitter’s users and Twitter itself under the U.S. Constitution include a right to disseminate such anonymous or pseudonymous free speech,” the lawsuit argues.

The lawsuit goes on to argue that many of these alt-government accounts, including @ALT_USCIS claim to be run by government workers, in this case employees of the U.S. Customs and Immigration Services. These users have taken to Twitter to voice their dissent with official government policies.

DHS summoned Twitter in March to unmask the user or users behind the account in question, but Twitter argued in its lawsuit that the means of this summons wouldn’t apply to this particular case. But the company went further than that, arguing that compliance with the summons would have a chilling effect on free speech, and effectively threaten other anonymous or pseudonymous Anti-Trump accounts.

The ACLU announced separately on Thursday that it will also go to court to protect the identity of @ALT_USCIS.

We're glad Twitter is pushing back. We'll be going to court to defend this user's right to anonymous speech. https://t.co/tqj5XrNvgn — ACLU National (@ACLU) April 6, 2017

The lawsuit is just the latest chapter in a contentious relationship between President Trump and Twitter. As a candidate, and continuing during his first few weeks in office, Trump has often used Twitter to circumvent traditional media organizations and reach an audience of millions.

However, Twitter and its executives have also been highly critical of the Trump administration, and particularly its ill-fated travel ban that targeted visitors and immigrants from seven Muslim-majority countries. And during the campaign, Twitter reportedly nixed plans for an ad campaign for then-candidate Trump at the last minute over plans to use a “crooked Hillary” emoji — a decision that campaign insiders called politically motivated, and that may have been a reason why Twitter’s executives didn’t get invited to a round-table with tech CEOs in December.