(CNN) -- A decision by Massachusetts prosecutors to subpoena the Twitter records of an Occupy Boston activist, as well as records linked to two Twitter hashtags, has free speech advocates up in arms, calling the move a violation of the First Amendment.
Suffolk County prosecutors demanded that Twitter hand over information posted on the social media website by user "Guido Fawkes," whose Twitter handle is @p0isAn0N, as well as information from the user behind @OccupyBoston and those who Tweeted #BostonPD or #d0xcak3, according to the document.
The ACLU gave CNN a copy of the subpoena.
"There is a constitutional right to write things on the Internet," said Peter Krupp, an attorney with the ACLU of Massachusetts who is representing the person using the Twitter handle Guido Fawkes. "There is a constitutional right to do that anonymously."
A Suffolk County Superior judge held a private hearing Thursday and impounded all documents pertaining to the case, according to Krupp, who said he could not divulge what happened during the session.
Krupp said he had requested that the meeting be open to the public.
"Secret court proceedings, particularly proceedings involving First Amendment issues, are troubling as a matter of both law and democracy," said Carol Rose, executive director for the ACLU of Massachusetts, in a statement.
Suffolk County District Attorney spokesman Jake Wark said that, since Thursday's court proceeding was not public, prosecutors could not comment on the hearing.
"We investigate and prosecute criminal acts," Wark said. "We have no interest in investigating political speech or political opinions."
In the subpoena, which was issued on Dec. 14, prosecutors requested that Twitter release to them "all available subscriber information," including IP address logs for the time period between Dec. 8 and Dec. 13 as part of an "official criminal investigation."
Those dates coincide with clashes between protesters and police in Boston's Dewey Square. Dozens of protesters were arrested after refusing to leave the public space after being ordered to do so by Boston's mayor, Thomas Menino.
Wark would not elaborate on the nature of the criminal investigation, but said Thursday that "no charges have been brought against any individual."
Prosecutors had requested that Twitter keep the subpoena confidential, but it was posted online last week and has been viewed by more than 20,000 people, Krupp said.
Subpoenaing Twitter records is becoming more common, according to lawyer Ethan Wall, of the Richman Greer law firm in Miami. Wall, who specializes in intellectual property litigation, said the practice could have "a chilling effect on free speech."
"We are in this information-accessible age where we can post anything and everything from anywhere on any device," Wall said. "Because it's so easy I don't think that people put the thought into how this might affect them personally, professionally or legally."
Henry J. Cittone of Cittone & Lindenbaum LLP, an intellectual property law firm in New York, said the most troubling part of the case is that prosecutors are subpoenaing hashtags, meaning that anyone who Tweeted or re-Tweeted #BostonPD or #d0xcak3 could be exposed during the investigation.
"It looks like authorities are trying to build lists of protestors by using hashtag information, as opposed to going after actual criminals -- which would have been the proper focus of such an investigation," Cittone said.
Krupp called the subpoena "wildly over-broad," pointing out that requesting records of two Twitter handles -- @p0isAn0N and @OccupyBoston -- could affect thousands.
"That's not just subscriber information for those accounts, but presumably followers," he said.
Twitter did not respond to requests for comment. A posting on its website says: "We may preserve or disclose your information if we believe that it is reasonably necessary to comply with a law, regulation or legal request; to protect the safety of any person; to address fraud, security or technical issues; or to protect Twitter's rights or property."