The shadowy underworld of Internet hackers was rocked by news Tuesday that one of the world's most-wanted and most-feared computer vandals has been an FBI informant for months and helped authorities build a case against five alleged comrades.

The FBI said it captured the legendary hacker known as "Sabu" last June, and he turned out to be Hector Xavier Monsegur, 28, a self-taught, unemployed computer programmer with no college education, living on welfare in public housing in Manhattan.

His exploits made him a hero to some in cyberspace until he made a rookie mistake -- he posted something online without cloaking his IP address, or computer identity -- and someone tipped off the FBI.

Soon after his arrest he pleaded guilty and began spilling secrets, leading to charges Tuesday against five people in Europe and the United States, including a Chicago man, and preventing more than 300 attacks along the way, authorities said.

Law enforcement officials said it marked the first time core members of the loosely organized worldwide hacking group Anonymous have been identified and charged in the United States.

Investigators said Monsegur and the other defendants were all associated with the group, and some were also part of the elite spinoff organization that Monsegur formed last May, Lulz Security, or LulzSec. "Lulz" is Internet slang for "laughs" or "amusement." Monsegur and the other defendants were accused in court papers of hacking into corporations and government agencies worldwide, including the U.S. Senate, filching confidential information, defacing websites and temporarily putting victims out of business. Authorities said their crimes affected more than 1 million people.

Prosecutors said that among other things, the hackers, with Monsegur as their ringleader, disrupted websites belonging to Visa, Mastercard and PayPal in 2010 and 2011 because the companies refused to accept donations to WikiLeaks, the organization that spilled a trove of U.S. military and diplomatic secrets.

His deal with prosecutors requires his full cooperation and testimony at any trial. In return, he gets leniency from a potential prison sentence of more than 120 years. He is free on $50,000 bail.

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