Anonymous -- a loosely associated hacktivist collective -- claimed its Operation Last Resort was responsible for the shutdown of the U.S. Justice Department Sentencing Commission's website early Saturday. The site was still loading slowly as of 8 p.m. EST.The Anonymous group said in a video posted on YouTube that it carried out the cyberattack in retaliation for the prosecution and subsequent suicide of free-speech activist Aaron Swartz, 26, on Jan. 11. (Both a link and the video appear at the bottom of this article.)At the time of his death, Swartz, an early architect of Reddit, was facing trial on charges that he employed the Massachusetts Institute of Technology's computer network to swipe more than 4 million articles from JStor, an online journal distribution service. Swartz could have been hit with a prison sentence of 31 years and fines of as much as $1 million."Two weeks ago today, a line was crossed," Anonymous said in the video. "Two weeks ago today, Aaron Swartz was killed. Killed because he faced an impossible choice. Killed because he was forced into playing a game he could not win -- a twisted and distorted perversion of justice -- a game where the only winning move was not to play."Anonymous added it is planning similar cyberattacks, saying: "The time has come to show the United States Department of Justice and its affiliates the true meaning of infiltration. The time has come to give this system a taste of its own medicine. The time has come for them to feel the helplessness and fear that comes with being forced into a game where the odds are stacked against them."Regarding its choice in targeting the Sentencing Commission's website, Anonymous said: "This website was chosen due to the symbolic nature of its purpose -- the federal sentencing guidelines which enable prosecutors to cheat citizens of their constitutionally guaranteed right to a fair trial, by a jury of their peers -- the federal sentencing guidelines which are in clear violation of the Eighth Amendment protection against cruel and unusual punishment. This website was also chosen due to the nature of its visitors."The hacktivist collective warned of its planned cyberattacks: "We have enough fissile material for multiple warheads. Today we are launching the first of these. Operation Last Resort has begun."Meanwhile, the FBI is investigating the cyberattack Saturday, according to Reuters."We were aware as soon as it happened and are handling it as a criminal investigation," Richard McFeely, a representative of the FBI's Criminal, Cyber, Response, and Services Branch, told the news agency via email. "We are always concerned when someone illegally accesses another person's or government agency's network."