(CNN) -- In anger over the recent death of an Internet activist who faced federal charges, hackers claiming to be from the group Anonymous threatened early Saturday to release sensitive information about the U.S. Department of Justice.
They claimed to have one such file on multiple servers ready for immediate release.
The hackers hijacked the website of the U.S. government agency responsible for federal sentencing guidelines, where they posted a message demanding the United States reform its justice system or face incriminating leaks to select news outlets.
The lengthy, eloquently written letter was signed "Anonymous."
Richard McFeely, executive assistant director of the FBI's Criminal, Cyber, Response, and Services Branch, said the bureau was immediately aware of the threat and is "handling it as a criminal investigation."
"We are always concerned when someone illegally accesses another person('s) or government agency's network," he said.
The U.S. Sentencing Commission said its website was brought down "temporarily" before it was restored later Saturday. "The commission's publications, training materials and federal sentencing statistics are again readily accessible to visitors to the site," it said in a statement.
The suicide of Internet activist Aaron Swartz on January 11 triggered the posting of the hackers' message to the web address of the sentencing commission, they said.
His death, which they blamed on the justice system, "crossed a line," the letter said.
A YouTube video accompanied the message, and made use of images from Cold War nuclear scenarios and games of strategy. The letter contained nuclear metaphors to refer to chunks of embarrassing information.
The hackers said they have obtained "enough fissile material for multiple warheads," which it would launch against the Justice Department and "its associated executive branches."
It gave the "warheads" the names of U.S. Supreme Court justices.
Anonymous accused the FBI of infiltrating its ranks and claimed the federal government is applying "highly disproportionate sentencing" to ruin the lives of some of its members.
Swartz, 26, was facing federal computer fraud charges and could have served 35 years in prison. Anonymous said he "was killed," because he "faced an impossible choice."
His family has issued a statement saying that federal charges filed over allegations that he stole millions of online documents contributed to Swartz's decision to take his own life. The files were mostly scholarly papers from the Massachusetts Institute of Technology.
Swartz's suicide has inspired a flurry of online tributes and mobilized Anonymous, the loosely defined collective of so-called "hacktivists" who oppose attempts to limit Internet freedoms. Both Swartz and Anonymous have been stark proponents of open access to information and open-source programming.
A review of a cached version of the USSC.gov website showed the Anonymous message on its homepage early Saturday.
Anonymous also posted an editable version of the website, inviting users to deface it as they pleased. Multiple pages -- not only the home page -- appeared to allow users to alter them.
The "warhead" names appeared as links, most leading to 404 error messages of pages not found, but some leading to pages of raw programming code.
The hackers said they chose the commission's website because of its influence on the doling out of sentences they consider to be unfair.
CNN's Carol Cratty, Jason Moon, AnneClaire Stapleton contributed to this report