The incident is the latest in a series of security-related problems at U.S. nuclear research facilities in the past few years and it comes at a time of heightened concern about nuclear material and technical expertise finding their way to terrorist organizations.

Los Alamos lab spokesman Kevin Roark says there has been no leak of critical data in this case and that it is being handled with an internal inquiry as opposed to a law enforcement investigation.

"What we conduct is called a security inquiry and that is for internal purposes only to determine who did what and when and once we have figured that out to try to decide what personnel actions might be appropriate," he said.

Mr. Roark says those personnel actions could include firing anyone found negligent.

But critics are not satisfied that an internal inquiry is sufficient. Peter Stockton, a former Department of Energy official who now works with the private watchdog group called The Project on Government Oversight, says the security breach was so serious that more drastic measures should be taken.

"There were four removable media with classified information on them that disappeared," he said. "Two were hard drives. One guy just took the two hard drives, put them in his pickup truck and drove off to another site with them. The two Zip drives, they still have not found. They did find the two hard drives."

Mr. Stockton says the current management team at Los Alamos, which is controlled by the University of California, should be removed and a new team brought in. He also suggests that such a sensitive and important national laboratory should not be using removable media (EDS: an electrical component to hold information) for classified information.

"They can go media-less in two to three months at most. It would cost them a few million dollars and it would save all this problem," he said.

He says a media-less system would have all computers with classified data linked to a central storage site by optical cables, with no removable storage media that could be taken off site.

Los Alamos spokesman Kevin Roark, however, says it is too early to call for drastic steps. He says the laboratory is already taking the proper measures to restore its security.

"The measure of an organization really should be how they handle their problems," he said. "I think we are handling our problems to the best of our ability. I think we are handling them pretty well. The history of this laboratory is solving very complicated problems. We have been doing that for more than six decades and we are going to do that in this case, too."