Twitter says the administration demanded they release private information about the account a month ago.
The US Justice Department declined to comment on the case.
"Twitter's actions in providing a platform for the dissemination of its users' speech - including its decision to permit the publication of pseudonymous speech - is fully protected by the First Amendment", Twitter said.
Twitter has sued the US Department of Homeland Security over its demands that the microblogging site unmask an anonymous anti-Trump account.
The company went on to argue any unmasking of the account would have a chilling effect on users critical of the Trump administration, and by extension, free speech. But Defendants have not come close to making any of those showings.
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The account in question is purportedly run by one or more current employees of the US Citizenship and Immigration Services (USCIS), a unit within Homeland Security.
Twitter said it received a summons on March 14 to produce the name or names of the account holder, saying the document asked the company "not to disclose the existence of this summons for an indefinite period of time". And even if Defendants could otherwise demonstrate an appropriate basis for impairing the First Amendment interests of Twitter and its users, they certainly may not do so using the particular investigatory tool employed here-which Congress authorized exclusively to ensure compliance with federal laws concerning imported merchandise-because it is apparent that whatever investigation Defendants are conducting here does not pertain to imported merchandise.
A copy of the summons filed with the lawsuit says the records are needed for an investigation to ensure compliance with duties, taxes and fines and other custom and immigration matters. Beyond that boilerplate language, the CBP Summons provides no justification for issuance of a summons targeting the @ALT_USCIS account.
Overall, the lawsuit proves that Twitter is willing to stand up to even the most powerful of legal opponents on behalf of its users. And the Court has likewise recognized that anonymity is often essential to fostering such political speech where, as here, the speaker could face retaliation or retribution if his or her real identity were linked to the speech. "We're going to fight for this user's right to remain anonymous".
"Alt" accounts for several agencies have since become increasingly popular, as turmoil within the White House and federal government at large has been played for satire online.