Internet rights attorneys appealed a US judge's order that Twitter must hand over data of three users in contact with the controversial website WikiLeaks.
The Electronic Frontier Foundation (EFF) and American Civil Liberties Union (ACLU) challenged the March 3 ruling on behalf of Icelandic parliamentarian Birgitta Jonsdottir, one of the Twitter users targeted by the decision.
The EFF and ACLU want Magistrate Judge Theresa Buchanan's decision overturned and WikiLeaks investigators to reveal any similar requests for information from other Internet firms.
"Except in very rare circumstances, the government should not be permitted to obtain information about individuals' private Internet communications in secret," said ACLU staff attorney Aden Fine.
"If the ruling is allowed to stand, our client might never know how many other companies have been ordered to turn over information about her, and she may never be able to challenge the invasive requests."
Besides Jonsdottir, the Twitter accounts belong to US computer researcher Jacob Appelbaum and Rop Gonggrijp, a Dutch volunteer for WikiLeaks.
The government request for information from Twitter became public because the California-based microblogging service notified them, according to the EFF.
In her decision, Buchanan rejected arguments that the grab for Twitter information violated freedom of speech and privacy.
She said the three "already made their Twitter posts and associations publicly available" and voluntarily provided information to Twitter pursuant to the website's privacy policy.
Buchanan also dismissed the argument that the order violated the Fourth Amendment to the US Constitution, which protects people against "unreasonable" searches.
When the trio relayed information to Twitter, they gave up "any reasonable expectation of privacy," she said.
"Services like Twitter have information that can be used to track us and link our communications across multiple services including Facebook and Gmail," said EFF Legal Director Cindy Cohn.
"The Magistrate's ruling that users have no ability to protect that information from the US government is especially troubling."
President Barack Obama's administration obtained a court order last year seeking information from the Twitter accounts as it considers action against WikiLeaks, which has released a flood of secret diplomatic documents.
WikiLeaks, which has strongly criticized the order, said that three Twitter users never worked for the site but that two helped make public a video that showed a 2007 US helicopter strike in Baghdad that killed several people.
The footage appeared to show the Apache pilots mistaking a camera carried by an employee of the Reuters news agency as a rocket-propelled grenade launcher.
WikiLeaks has since angered US authorities by posting secret documents on the wars in Afghanistan and Iraq and releasing a slew of internal correspondence among US diplomats around the world.
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