Twitter has defied a US government request for records that could identify users behind an account opposed to President Donald Trump, and is challenging that order in court.
The company filed its lawsuit on Thursday in a San Francisco federal court against the federal Department of Homeland Security and its Customs and Border Protection office, charging that their efforts to "unmask" the people behind the account violate the First Amendment.
Twitter said its users have a constitutional right to disseminate such "anonymous and pseudonymous political speech". It declined to comment beyond the lawsuit. DHS also refused to comment.
The account in question is @ALT_uscis, a reference to the US Citizenship and Immigration Services office. The account described its users to the Associated Press in February as employees and former employees of the agency.
In a interview on Thursday, an American Civil Liberties Union lawyer now representing the account declined to discuss anything about the person or people currently involved in the account being targeted by the Trump administration.
Documents supporting the Twitter account holder's right to speak anonymously online will be filed in the next few days, said Esha Bhandari, a staff lawyer for the ACLU.
"We think it's very important for the user's interests to be represented as well," Ms Bhandari said.
"The First Amendment requires the government to have a very compelling reason for unmasking someone's identity. That is important or people would be chilled from speaking out, particularly when they are speaking out against the government."
The government has so far not specified a reason for wanting to know the identity or identities behind the Twitter handle.
In the two months of its existence, the account has been critical of the Trump administration's immigration policies and "highlighted what the user views as a history of waste and mismanagement within USCIS and DHS", according to the lawsuit.
It is one of dozens of rogue Twitter accounts purporting to represent current or former federal employees that have sprung up since Mr Trump took office.
Other such "alternative" - or "alt" - accounts include @Alt_CDC for the Centers for Disease Control and Prevention and @AltUSEPA for the Environmental Protection Agency. The latter dubs itself "The Unofficial 'Resistance' team of US Environmental Protection Agency".
Twitter said it received a summons on March 14 demanding that it provide records which would unmask the @ALT_uscis account, such as user names, account login phone numbers, mailing addresses and computer IP addresses.
The faxed summons - from an agent at Customs and Border Protection - ordered Twitter to produce the records on March 13, the day before Twitter received it, according to the suit.
In its lawsuit, Twitter claimed the government cannot compel the company to disclose users' identities without first meeting several tests. It must prove that a criminal or civil offence has been committed, that it is not asking for information with the intent of suppressing free speech, and that the interests of the investigation outweigh the First Amendment rights of Twitter and its users, Twitter said.
But the federal agencies and other defendants "have not come close" to demonstrating any of this, the lawsuit states.
"We are very heartened that Twitter is standing up for its users and making such full-throated defence of their free-speech rights," Ms Bhandari said.
This is not the first time Twitter has filed a suit in defence of its and its users' First Amendment rights.
In 2012, the company fought back against a court order compelling it to hand over basic user information and tweets from an Occupy Wall Street protester. The protester later pleaded guilty to disorderly conduct. The posts had been public.
In 2014, Twitter sued the US government (under President Barack Obama) seeking to publish its full "transparency report" outlining government requests for information.