The US government has subpoenaed the social networking site Twitter for personal details of people connected to Wikileaks, court documents show.
The US District Court in Virginia said it wanted information including user names, addresses, connection records, telephone numbers and payment details.
Those named include Wikileaks founder Julian Assange and an Icelandic MP.
The US is examining possible charges against Mr Assange over the leaking of classified diplomatic cables.
Reports indicate the Department of Justice may seek to indict him on charges of conspiring to steal documents with Private First Class Bradley Manning, a US Army intelligence analyst.
Mr Manning is facing a court martial and up to 52 years in prison for allegedly sending Wikileaks the diplomatic cables, as well military logs about incidents in Afghanistan and Iraq and a classified military video.
'Given a message'
According to the court order issued on 14 December by the District Court for the Eastern District of Virginia, the US Attorney's Office has provided evidence to show that the information held by Twitter is "relevant and material to an ongoing criminal investigation".
The San Francisco-based website was given three days to respond was also told not to disclose that it had been served the subpoena, or the existence of the investigation.
However, the same court removed those restrictions on Wednesday and authorised Twitter to disclose the order to its customers.
The subpoena requested the details of Mr Assange, Pfc Manning and Icelandic MP Birgitta Jonsdottir, as well as Dutch hacker Rop Gonggrijp and US programmer Jacob Appelbaum, both of whom have previously worked with Wikileaks.
The information sought includes mailing addresses and billing information, connection records and session times, IP addresses used to access Twitter, email accounts, as well as the "means and source of payment".
Mr Assange condemned the court order on Saturday, saying it amounted to harassment.
"If the Iranian government was to attempt to coercively obtain this information from journalists and activists of foreign nations, human rights groups around the world would speak out," he said in a statement.
The order was unsealed "thanks to legal action by Twitter", he added.
And Mr Assange's lawyer, Mark Stephens, said the US government was attempting to intimidate people.
"It's a great disappointment that the department of justice has stopped playing lawyers and started playing politics," he told BBC News.
Twitter has declined to comment on the claim, saying only: "To help users protect their rights, it's our policy to notify users about law enforcement and governmental requests for their information, unless we are prevented by law from doing so."
Ms Jonsdottir, who until recently was a vocal supporter of Wikileaks, revealed on Friday that the department of justice had asked Twitter for her personal details and all of her tweets since November 2009.
She said she had 10 days to appeal against the subpoena.
Ms Jonsdottir wrote on her Twitter feed: "USA government wants to know about all my tweets and more since 1 November 2009. Do they realise I am a member of parliament in Iceland?"
She said that she would call Iceland's justice minister to discuss the request.
"I think I am being given a message, almost like someone breathing in a phone," she said.
Ms Jonsdottir was the chief sponsor of the Icelandic Modern Media Initiative (IMMI) law, which made Iceland an international haven for investigative journalism and free speech.
She has said she helped to produce a video for Wikileaks showing a US Apache helicopter shooting civilians in Iraq in 2007.
The classified video, released by Wikileaks last April, brought the whistle-blowing website to the world's attention.
The website's founder, Julian Assange, is currently fighting extradition from the UK to Sweden, where he is wanted for questioning as part of an inquiry into alleged sex offences.
Ms Jonsdottir reportedly left Wikileaks late last year after she argued unsuccessfully that Mr Assange should take a low-profile role until his legal troubles were resolved.