A computer "hacker" who penetrated systems at the Lawrence Livermore National Laboratory eight times in the past week has given himself "super-user" status and has the power to roam through some unclassified systems at will and destroy data.

Officials at the lab, where the nation's nuclear weapons and components of the "Star Wars" defense system are designed, said the most recent incident occurred Wednesday night. The hacker has exploited holes in the Unix operating system, the basic program that controls many of the nation's research computers.The Cray supercomputers used to design nuclear bombs are physically and electronically isolated from the main systems and are not involved in the breach of computer security, officials said.

While lab security officials are working to plug the holes that have allowed the hacker to obtain unauthorized access, they are also deliberately leaving some computers vulnerable in an attempt to catch the hacker much the same way police use telephone taps.

Officials at the laboratory said the hacker now has the power to go through some unclassified files and destroy other data.

"Initially, I just assumed it was kids joy riding out there in the network," said Chuck Cole, the lab's chief of computer security. "I get more worried as time goes by. He could be destroying files if he chose to do that. What worries me is where is he that we don't know about?"

Cole said the hacker broke into the systems five times last weekend and twice more Wednesday morning, coming in through a network that links the computers of various universities and government research facilities.

On Wednesday night, he made contact directly through a telephone line, using a secret "dial up" number. Once inside the system, he used a loophole in the Unix system to assign himself a "super-user" code, "FTP anonymous demon."

While using the phone line gave the hacker more power within the system, it could also make him a little easier to track down, lab spokesman Ron Tunnis said.

"It's like a chess game," Tunnis said. "They try to block him in one machine and he tries to come in through another one."