March 9, 2012, 12:12 p.m. ET.

Hacker Involved in Attacks Dating to 1999, Prosecutors Say .

By CHAD BRAY

NEW YORK—A leading computer hacker who became an informant for the government last year allegedly carried out a variety of online attacks from as early as 1999 and allegedly committed a variety of other crimes for which he won't be prosecuted because of his cooperation, it emerged on Friday.

Hector Xavier Monsegur, known in the hacking world as "Sabu," allegedly infiltrated and defaced numerous websites over the years and once attempted to sell four pounds of marijuana and used a former employer's credit card to make $15,000 in purchases, according to a plea agreement made public on Friday.

Federal prosecutors aren't going to prosecute Mr. Monsegur, 28 years old, for those crimes, following his cooperation in an investigation that led to charges unveiled earlier this week against a group of men allegedly behind "LulzSec"—a secretive collective of alleged cybercriminals who targeted companies, governments and individuals around the world.

Mr. Monsegur, who was arrested on June 7 last year, pleaded guilty two months later to 12 criminal charges, including three counts of conspiracy to engage in computer hacking, conspiracy to commit bank fraud and aggravated identity theft. He faces more than 122 years in prison. A lawyer for Mr. Monsegur declined to comment on Friday.

Mr. Monsegur began cooperating with federal agents shortly after his arrest, ferreting out details about security vulnerabilities in corporate and other websites.

"Since literally the day he was arrested, the defendant has been cooperating with the government proactively," Assistant U.S. Attorney James Pastore said at an Aug. 5, 2011, bail hearing. "He has been staying up sometimes all night engaging in conversations with co-conspirators that are helping the government build cases against those co-conspirators," Mr. Pastore added.

But the plea agreement with Mr. Monsegur was unusual in that it applies to all 94 U.S. Attorney's offices in the nation, prosecutors said when he pleaded guilty on Aug. 15, according to a transcript.

LulzSec, formed last May, claimed responsibility for a series of brazen online attacks including hacking computer servers of television network PBS in retaliation for a "Frontline" episode about WikiLeaks and stealing personal information from about 100,000 customers of hacked Sony Pictures.

In addition to the Sony and PBS attacks, LulzSec has claimed responsibility for attacks on the U.S. Senate and InfraGard, an affiliate of the Atlanta chapter of the FBI. Those attacks were all cited in Tuesday's charging documents.

Write to Chad Bray at chad.bray@wsj.com

Hacker Involved in Attacks Dating to 1999, Prosecutors Say - WSJ.com