A self-described anarchist and "hacktivist" from Chicago pleaded guilty Tuesday to charges he illegally accessed computer systems of law enforcement agencies and government contractors.

"As part of each of these hacks, I took and decimated confidential information stored on computer systems websites used by each of the entities," Jeremy Hammond told a judge in federal court in Manhattan. "For each of these hacks, I knew what I was doing was against the law."

Prosecutors had alleged the cyber-attacks were carried out by Anonymous, the loosely organized worldwide hacking group that stole confidential information, defaced websites and temporarily put some victims out of business. Hammond was caught last year with the help of Hector Xavier Monsegur, a famous hacker known as Sabu who later helped law enforcement infiltrate Anonymous.

A criminal complaint had accused Hammond of pilfering information of more than 850,000 people via his attack on Austin, Texas-based Strategic Forecasting Inc., a publisher of geopolitical information also known as Stratfor. He also was accused of using the credit card numbers of Stratfor clients to make charges of at least $700,000. He allegedly bragged he even snared the personal data of a former U.S. vice president and one-time CIA director.