But charges by the British police link Mr. Cleary to a hacking group called Lulz Security, or LulzSec, which has been on an Internet crime spree in recent weeks, attacking Web sites and computer networks including those of the United States Senate, the Central Intelligence Agency and Sony.
The British tabloids have been quick to cast Mr. Cleary as the young criminal mastermind behind LulzSec, calling him “Hack the Lad” in front-page headlines. His mother, Rita, has said her son is highly intelligent but has a history of mental illness, including agoraphobia. His lawyer, Ben Cooper, described Mr. Cleary as “a vulnerable young man.”
Though it is not clear how much notoriety he deserves, Mr. Cleary’s arrest has made him a focus of the public fascination with a wave of computer hacking cases, carried out by amorphous online collectives.
The police say Mr. Cleary is guilty of illegally using a computer to perform denial of service attacks — bombarding Web sites with so many automated messages that they shut down. They say his targets were organizations including the British Serious Organized Crime Agency.
In the hierarchy of computer hacking, the accusations against Mr. Cleary and the actions of LulzSec fall broadly into the category known as hacktivism. Hackers of this type are not motivated by money, but are mainly interested in protesting against or antagonizing their targets, or in showing off technical skills.
Hacktivists, according to computer security experts, are a different breed from mainstream cybercriminals, who seek financial gain. Such criminals, for example, manipulated Citigroup’s Web site to steal the personal information of credit card holders.
The third category, experts say, are warriors, either working in the “cybercommands” of governments like those of the United States and other countries, or for mercenary or terrorist groups. They defend computer networks, power grids and state secrets of their own country, while devising tactics to attack enemies.
Hacktivists tend to portray their activities as digital sit-ins, a form of protest. But security experts say their attacks often cause real damage to computer networks and financial losses. LulzSec has been more aggressive than most, and more brazen in its choice of targets.
“This is organized criminal activity that is typically distributed across many different countries,” said Mark Rasch, a former prosecutor in the Justice Department, who is director of security for CSC, a computer services company. “It’s a serious crime.”
On Thursday evening LulzSec released what it said were hundreds of internal documents from the Arizona Department of Public Safety, including material related to border patrol and counterterrorism operations. It said it was taking aim at the agency because of Arizona’s anti-immigrant policies. A Department of Public Safety spokesman, Capt. Steve Harrison, said the documents appeared to be authentic but were sensitive, not confidential.
Hacking has been a pursuit of mischievous young men — and they are nearly all men — since shortly after computers were invented. But the Internet made it an increasingly international pursuit. The intruders quickly became power users of online bulletin boards and Internet chat software, using those tools to communicate and organize activities.
“Hackers were among the first to figure out the benefits of social networking,” said Alan Brill, a senior managing director of Kroll, a security consulting firm.
The far-flung hacker networks present a formidable challenge for law enforcement. But in recent years, they and prosecutors have more and more formed their own international networks of communication, sharing information across borders. Mr. Cleary’s arrest, for example, involved cooperation between Scotland Yard and the F.B.I.
LulzSec, on a Twitter feed that it uses to communicate with more than 250,000 followers, has said that Mr. Cleary is “at best mildly associated with us.” The group did not respond to a Twitter message seeking comment for this article.
LulzSec, experts say, is a splinter group from Anonymous, another online hacking collective. Anonymous is best known for its attacks last year in support of WikiLeaks, led by Julian Assange. The group went after the Web sites of companies like MasterCard and PayPal, which had refused to process donations to WikiLeaks after it disclosed confidential diplomatic cables.
Earlier this year, said Barrett Brown, a former Anonymous activist, “some of the most prominent leaders and hackers broke off and are now LulzSec.”
The two hacker groups certainly strike different poses. LulzSec’s statements and its actions display a spirit of exuberant anarchic glee. Lulz, in essence, means mean-spirited laughter, and LulzSec’s Web site describes the group as “a small team of lulzy individuals who feel the drabness of the cybercommunity is a burden on what matters: fun.”
The group is strongly antagonistic to the media. When a TV journalist for Russia Today asked for an interview, she was told it would be granted only if she and her producer wore shoes on their heads and wrestled in mud while singing. They declined.
There seems to be far less glee in the Anonymous culture. In a YouTube video describing the group, a voice intones: “There is no control, no leadership, only influence. The influence of thought.” Later, the video adds that Anonymous’s actions have “brought justice to our world.”
LulzSec’s exploits have riled others in the hacker world who object to its activities, particularly exposure of personal information of innocent Internet users. Those people are now working to stop LulzSec by investigating its members’ identities and providing information to the F.B.I.
The core LulzSec group, according to Mr. Brown, the former Anonymous activist, numbers between five and 10. Mr. Brown said the members he had dealt with — known by online nicknames like Topiary and Sabu — are mostly men in their early 20s.
Mr. Brown said he had dealt with Mr. Cleary, and that he believed — contrary to LulzSec’s statement — that he was involved with the group. But a person involved with Anonymous, who declined to be named for fear of prosecution, said Mr. Cleary was peripheral.
On Thursday the court agreed to delay Mr. Cleary’s application for bail while police investigated.
Hacker networks and their activities are murky by design, said Bruce Schneier, chief security technology officer of the British company BT Group. LulzSec, Mr. Schneier said, “is a badge, a name you call each other if you’re one of the cool hacker kids now.”
Riva Richmond contributed from New York.
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