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

The Sentencing Commission site, www.ussc.gov, was shut down early Saturday. The commission is an independent agency within the judicial branch that helps craft sentencing guidelines.

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' January 11 suicide.

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."

(This story removes reference in first paragraph to Sentencing Commission as part of U.S. Justice Department in Jan. 26 story)

(Reporting by Deborah Zabarenko; Editing by Vicki Allen and Bill Trott)