A group belonging to the Anonymous hacker collective is poised to publish a set of emails containing details about the Haditha massacre, in which 24 unarmed Iraqi people were killed by US Marines.

The hacktivists announced on Pastebin that they would dump a torrent of a few gigabytes of email containing "detailed records, transcripts, testimony, trial evidence, and legal defence donation records". The group hacked the email account of Puckett Faraj, a law firm that represents Staff Sgt Frank Wuterich, who is charged with leading a group of marines in Haditha.

A further statement on Pastebin contains details of an email exchange between associates of the law firm after the hack. A spokesperson for the law firm confirmed that the website was down but did not confirm its hacking.

Wuterich, 31, admitted one count of dereliction of duty but manslaughter charges were dropped as part of a plea deal, AFP reported.

Wuterich, who led an eight-man squad whose other members have all been dismissed, was sentenced to 90 days' confinement and reduced in rank to private but will not go to jail under the deal with prosecutors.

"Can you believe this [expletive deleted] had his charges reduced to involuntary manslaughter and got away with only a pay cut?" reads the statement on Pastebin. "Meanwhile Bradley Manning who was brave enough to risk his life and freedom to expose the truth about government corruption is threatened with life imprisonment."

Manning is the US soldier accused of leaking classified US documents to WikiLeaks.