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Twitter sued the federal government on Thursday to protect the identity of the person or people behind an anonymous account highly critical of Customs and Border Protection.

In an action filed in U.S. District Court in San Francisco, lawyers for Twitter disclosed a summons the social media company received last month from CBP and its parent agency, the Department of Homeland Security, demanding that the company turn over everything it knows about the anonymous account @ALT_uscis.

The summons, which was sent to Twitter on March 14, seeks user names, login records, phone numbers, physical addresses and IP addresses of anyone associated with the account, which tweets sharply critical comments about U.S. immigration policy and President Donald trump under the guise of disgruntled employees of U.S. Citizenship and Immigration Services.

A section of a summons sent to Twitter demanding user names, login records, phone numbers, physical addresses and IP addresses of anyone associated with the account @ALT_uscis.

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The CBP summons doesn't state any reason for the demand — a key omission that is at the heart of Twitter's response.

Twitter said CBP was obligated to demonstrate that some criminal or civil offense had been committed and that outing the account holders' identities was the least restrictive way to advance the investigation.

The American Civil Liberties Union, which told NBC News that is representing the account holder, similarly argued that the government hadn't presented a strong justification for demanding the information.

"In this case the government has given no reason at all, leading to concerns that it is simply trying to stifle dissent," said Nathan Freed Wessler, a lawyer with the ACLU's Speech, Privacy and Technology Project.

In any event, Twitter's lawyers wrote, "Permitting CBP to pierce the pseudonym of the @ALT_USCIS 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."

DHS Press Secretary Jenny Burke told NBC News: "As a matter of policy, we do not comment on pending litigation."

Nicholas Pacilio, a spokesman for Twitter, likewise said: "We're not providing comment beyond what's in the filing."

@ALT_uscis, itself, however, had plenty to say. It tweeted out the First Amendment to the U.S. Constitution:

Then it drew a comparison between CBP's investigation of it and the "unmasking" of people referred to intelligence reports on alleged Russian hacking:

Last week : Unmasking is outrageous when it comes to Russian collusion.

Today: Unmask @alt_uscis because customs import code. — ALT?? Immigration (@ALT_uscis) April 6, 2017

And it added this joke: