WASHINGTON (Reuters) - Hackers sympathetic to the late computer prodigy Aaron Swartz claimed on Saturday to have infiltrated the website of the U.S. Justice Department's Sentencing Commission, and said they planned to release government data.

The Sentencing Commission site, www.ussc.gov , was shut down early Saturday.

Identifying themselves as Anonymous, a loosely organized group of unknown provenance associated with a range of recent online actions, the hackers voiced outrage over Swartz' suicide on January 11.

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 more than 4 million articles from JSTOR, an online archive and journal distribution service.

Swartz had faced a maximum sentence of 31 years in prison and fines of up to $1 million.

The FBI is investigating the attack, according to Richard McFeely, of the bureau's Criminal, Cyber, Response, and Services Branch.

"We were aware as soon as it happened and are handling it as a criminal investigation," McFeely said in an emailed statement. "We are always concerned when someone illegally accesses another person's or government agency's network."

(Reporting by Deborah Zabarenko; Editing by Vicki Allen)