A hacker who browsed for weeks through the unclassified computer system of a top federal nuclear weapons laboratory has been identified and has apologized to authorities.

No arrests have been made, officials said Saturday. FBI and lab officials refused to identify the hacker."In a communication, he regretted the difficulty caused by his actions and agreed to stop accessing LLNL computers," said Jeff Garberson, spokesman at the Lawrence Livermore National Laboratory.

Lab officials had pleaded through local media for the hacker to contact them. The pleas led to several phone calls identifying possible suspects, but it was unclear if the identity of the hacker was discovered through a tip.

A spokesman said earlier in the week that the hacker did not invade the lab's top secret nuclear weapons research computers, which are physically and electronically separated from the unclassified system.

The lab, run by the University of California at Berkeley, is one of the nation's two centers for design, manufacture and testing of nuclear weapons. The other is at Los Alamos, N.M.

The FBI is investigating the case, spokesman John Holford said.

Garberson said because criminal charges may be filed, lab officials will not provide any further information.

The hacker first appeared on the lab's Internet system about two weeks ago. The system connects research facilities and universities nationwide.

The hacker accessed various computers at the lab and even gave himself "super user" status so he could roam through files.