Monsegur pleaded guilty in August to charges that included conspiracy to commit hacking, admitting he obtained dozens of credit card numbers online and gave them to others or used them to pay his bills. His lawyer, Philip L. Weinstein, declined to comment Tuesday.
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.
Also charged with conspiracy to commit computer hacking were Ryan Ackroyd, 25, of Doncaster, England; Jake Davis, 19, of Lerwick, Scotland; Darren Martyn, 25, of Galway, Ireland, and Donncha O'Cearrbhail, 19, of Birr, Ireland.
Davis' lawyer, Adel Buckingham, declined comment. Contact information for the other European defendants' lawyers could not immediately be located.
Hammond, who was arrested Monday, appeared before a federal judge in Chicago and was ordered transferred to New York.
Defense attorney Jim Fennerty described Hammond as compassionate, saying he had rallied against plans to hold the 2016 Olympics in Chicago because he felt it would hurt low-income people and had protested against neo-Nazi groups.
"He's concerned about people and issues — that's why I like him," Fennerty said.
In July, when LulzSec's attacks were grabbing world headlines, someone alleged that Sabu was Monsegur and posted personal details about him on the Internet. Sabu took to Twitter to deny it.
Barrett Brown, a former journalist who became closely associated with Anonymous, said Sabu's cooperation with the FBI could do serious damage to Anonymous.
"He was an admired Anon," he said. "He's been a leader. People came to him with information. God knows what else he told them."
Associated Press writer Satter reported from London. Also contributing were Associated Press writers Larry Neumeister and David B. Caruso in New York and Michael Tarm in Chicago.
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