Law enforcement officials believe two teenage boys from a small town in Northern California were among a group of hackers who wormed their way into several U.S. government computer systems earlier this month, sources close to the investigation said yesterday.

The high school sophomores, aged 16 and 17, and other members of the group are suspected of intruding into at least 11 sensitive computer systems at U.S. military installations and dozens of systems at other government facilities, including federal laboratories that perform nuclear weapons research, the sources said.

Although the computers contained only unclassified information, Deputy Defense Secretary John J. Hamre told reporters Wednesday the military intrusions constituted "the most organized and systematic attack" on U.S. networks discovered by American authorities and thus aroused concern among officials worried about the possibility of electronic sabotage as a means of terrorism or warfare.

Government officials said they are not yet sure of the group's motives, but fellow students speculated the two youths were in it for thrills. Some law enforcement sources speculate that the group may have been competing to enter different systems.

Although the hackers are not believed to have seriously tampered with any of the systems they entered, they obtained powerful administrator privileges on some that could have let them perform tasks including deleting files and reading passwords, sources said.

FBI agents on Wednesday night raided the boys' homes, 70 miles north of San Francisco at Cloverdale in the California wine country, catching one of them in the act of entering a Pentagon computer, the sources said. Agents confiscated the boys' computers but did not arrest them. The two were not identified.

George Grotz, an FBI spokesman in San Francisco, confirmed that agents conducted searches at two residences in Sonoma County, which encompasses Cloverdale, "based upon a computer intrusion case we are working on," but declined to elaborate. He said the agents seized "some hardware, software and printers and other computer equipment."

The timing of the intrusions, as the United States was stepping up deployments of troops and equipment to the Persian Gulf region, particularly concerned Pentagon officials.

Four Navy and seven Air Force systems, at sites around the United States and Okinawa, are known to have been targeted. The systems perform largely logistical and administrative functions, but officials warned that tampering with even that data could potentially disrupt military operations.

The group also entered systems at several universities and federal research facilities, including the Oak Ridge National Laboratory, the Brookhaven National Laboratories, the University of California at Berkeley and the Massachusetts Institute of Technology's fusion labs, said William G. Zane, owner of Netdex Internet Services Inc., a Santa Rosa, Calif.-based Internet service provider. Zane said his service was intruded by the group and used as a launching pad for many of the attacks.

The group's activities first were noticed by Zane in mid-January, when his staff discovered an intruder who had given himself administrator, or "root," privileges, he said. Zane said he notified the FBI and the Computer Emergency Response Team at Carnegie Mellon University in Pittsburgh, which distributes computer security alerts.

Instead of kicking the hackers off his service, Zane said he consulted with the FBI and decided to install software that would monitor the group's activities, reports of which he said he provided to agents.

"We decided to take a little risk," he said. "We let them play for a little while. We gave them enough rope and let them hang themselves."

Zane said the hackers used another Internet service provider to connect to his computers. From his system, he said the group launched "hundreds of attacks."

"We were seeing hundreds of [Internet] addresses that would go by that had been scanned and cracked," he said.

One law enforcement source, however, said investigators have not yet been able to confirm the full scope of the group's intrusions. "We shouldn't jump to the conclusion that hundreds of computers were entered," the source said.

Zane refused to say what tactics the hackers used to enter his system and others. "They had a very sophisticated set of [software] tools," he said.

He said at least one of the hackers discovered last Saturday that the Netdex staff had been monitoring the group, resulting in a frantic scramble to copy the group's files for the FBI before the hacker deleted data on the computer in question.

The two students are 10th graders at Cloverdale High School, a 450-student public school. Gene Lile, the principal, said no federal or state authorities have yet notified the school of any wrongdoing, but the incident was the subject of almost every conversation throughout the school day yesterday.

Students who know the two boys fiercely defended them, describing them as "just normal, everyday kids" who have a strong interest in computers. "They were just doing it for the thrill of getting in. I know them well enough to know that they wouldn't do anything purposefully wrong," said Lissa Tryer, another 10th grader.

"It makes me mad. They're not twisted. These are just two nice guys," said another girl, who declined to give her name.

Joe Simao, a senior at Cloverdale, said he wasn't surprised to hear of the hacking incident. Simao said he helped one of the teens set up a computer operating system called Linux, which is based on Unix, a widely used operating system on the Internet. "They started to get into hacking lately," Simao said. "I heard [one of the teens] say, `Yeah, I've hacked into MIT and UC Berkeley.` "

Simao said he believes the pair did not have the expertise to invent new techniques for breaking into systems, but said they had become frequent users of sites on the Internet where hackers post descriptions of how to break into computer networks. "It's like some want to drive fast cars; others want to use drugs; and some want to hack," Simao said.

John Hudspeth, who teaches technology at Cloverdale and oversees a lab with 40 desktop computers, said that both boys have a sincere interest in technology. "They're good kids who have helped me out quite a bit," he said. "They've put a lot of time into helping set up the systems in this lab."

But Zane contended the attacks should not be minimized. "This was a massive set of intrusions," he said. "You don't need to crack 11 machines to brag that you've entered the Pentagon."

Corcoran reported from Cloverdale, Chandrasekaran from Washington. Staff writer Roberto Suro in Washington contributed to this report.