Hacker group Anonymous said it had disabled the US Sentencing Commission's website in revenge for the death of internet freedom advocate Aaron Swartz, and vowed to release government data.
The website of the commission, an independent agency of the US Justice Department involved in sentencing, was apparently hacked into early Saturday.
Anonymous threatened to make public the encryption keys to files that could potentially embarrass judges and other federal employees, saying it acted in protest at the Justice Department's alleged mishandling of Swartz's case.
Swartz was facing 35 years in prison and a $US1 million ($A960,661) fine for breaking into a closet at the Massachusetts Institute of Technology to plug into the computer network.
He downloaded millions of academic journal articles he had allegedly planned to distribute for free.
Friends and relatives have accused overzealous prosecutors of contributing to the 26-year-old's suicide. He was found hanged in his New York apartment earlier this month.
Swartz was just 14 when he co-developed the RSS feeds that are now the norm for publishing updates online and went on to help launch social news website Reddit.
"As a result of the FBI's infiltration and entrapment tactics, several more of our brethren now face similar disproportionate persecution, the balance of their lives hanging on the severely skewed scales of a broken justice system," Anonymous said in a video posted on YouTube.
The hackers said they had infiltrated several US government computer networks and copied secret data that they could release.
Likening the data to a nuclear weapon, Anonymous said it had "enough fissile material for multiple warheads" it could launch against the Justice Department and agencies that the group says intrude on individual liberties.
The FBI said in a statement that it was investigating the attack.
"We were aware as soon as it happened and are handling it as a criminal investigation," said Richard McFeely, of the bureau's Criminal, Cyber, Response and Services Branch.
The Anonymous operation came just days after US Homeland Security Secretary Janet Napolitano warned of a looming "cyber 9/11" that could especially target critical infrastructure planks such as water, electricity and gas lines.
The loosely affiliated network of hacktivists, has attacked sites around the world, including those of MasterCard and Visa, the US Justice Department as well as the Tunisian and Yemeni governments.