WASHINGTON -- The hacker-activist group Anonymous says it hijacked the Web site of the U.S. Sentencing Commission to avenge the death of Aaron Swartz, an Internet activist who committed suicide. The FBI is investigating.The Web site of the commission, an independent agency of the judicial branch, was taken over early Saturday and replaced with a message warning that when Swartz killed himself two weeks ago "a line was crossed." The hackers say they've infiltrated several government computer systems and copied secret information that they now threaten to make public. Family and friends of Swartz, who helped create Reddit and RSS, say he killed himself after he was hounded by federal prosecutors. Officials say he helped post millions of court documents for free online and that he illegally downloaded millions of academic articles from an online clearinghouse. The FBI's Richard McFeely, executive assistant director of the Criminal, Cyber, Response, and Services Branch, said in a statement that "we were aware as soon as it happened and are handling it as a criminal investigation. We are always concerned when someone illegally accesses another person's or government agency's network."