The Anonymous group claims it hacked the website of the U.S. Justice Department's Sentencing Commission to avenge the death of Internet activist Aaron Swartz.
The Sentencing Commission site, www.ussc.gov , was shut down early Saturday.
The hackers claim that they've broken into several government computer systems and copied secret information that they now threaten to make public.
In a video posted online, the hackers criticized the government's prosecution of Swartz, who had been facing trial on charges that he used the Massachusetts Institute of Technology's computer networks to steal millions of academic articles.
Aaron Swartz, who helped create Reddit and RSS, is said to killed himself after he was hounded by federal prosecutors.