Monsegur provided agents with information that helped them arrest several suspects on Tuesday, including two men from Great Britain, two from Ireland and an American in Chicago, according to FoxNews.
"This is devastating to the organization," an FBI official told FoxNews. "We're chopping off the head of LulzSec."
Monsegur, an unemployed father of two, allegedly led the loosely organized group of hackers from his apartment in a public housing project in New York. After his arrest he reportedly pleaded guilty Aug. 15 to 12 hacking-related charges. Documents in his case are expected to be unsealed in New York's Southern District Court on Tuesday.
"They caught him and he was secretly arrested and now works for the FBI," a source told FoxNews.com.
Those arrested on Tuesday include Ryan Ackroyd, aka Kayla, Jake Davis, aka Topiary, from London; Darren Martyn, aka "pwnsauce," and Donncha O'Cearrbhail, aka palladium, from Ireland; and Jeremy Hammond, aka Anarchaos, from Chicago.
Hammond, a member of Anonymous -- a group loosely affiliated with LulzSec -- is believed to be the main actor behind the hack of U.S. security company Stratfor last December, which resulted in the seizure of more than five million company e-mails, customer credit card numbers and other confidential information. The secret-spilling site WikiLeaks has begun to publish the e-mail with media partners around the world.
Ackroyd was allegedly Monsegur's top deputy and was responsible for uncovering vulnerabilities in the U.S. Senate's computer systems, according to FoxNews sources.