Steeped in history going back to the birth of the nuclear bomb, the government's Los Alamos National Laboratory (search) has been dogged in recent years by one embarrassment after another, from credit card fraud and allegations of espionage to disappearing files and safety lapses.

The latest flap: two missing computer disks containing nuclear secrets.

All classified work at the laboratory in the hills of northern New Mexico has been ordered stopped as some of the country's smartest nuclear weapons scientists and engineers search for the missing disks.

At the Energy Department (search), senior officials are steaming at what they view as yet another security foul-up at the facility where 61 years ago scientists put together the world's first atomic bomb.

Investigators have been stymied on the whereabouts of the two computer disks, known to the scientists as "classified removable electronic media." Energy Secretary Spencer Abraham (search) said the disappearance reflects "a widespread disregard for security" by lab officials.

"This is absolutely unacceptable," he fumed, ordering his top deputy, Kyle McSlarrow, to the laboratory to get to the bottom of it all.

Stretched across 43 square miles, the Los Alamos weapons lab employs more than 10,700 people, two-thirds of them working for the University of California, which has managed the facility since it was created as part of the World War II Manhattan Project that launched the age of nuclear weapons.

It's been 18 months since Peter Nanos, a retired vice admiral, took over as lab director after a scandal involving lab employees using laboratory credit cards to buy personal items including -- as alleged but still in some dispute -- a new Mustang automobile.

"We are not a bunch of crooks," Nanos told lab workers his first day on the job. "The trouble is I can't prove it."

Last week, faced with the latest computer disk flap, Nanos blamed "a small number" of people who cannot follow the rules and who again have "brought disrepute to Los Alamos."

No one has said what is on the disks and it's possible they may have been destroyed without anyone bothering with the required paperwork. To increase security, the lab has begun a program to phase out the use of removable disks from all its classified computers.

But it is far from the first embarrassing incident at the lab.

The Los Alamos nuclear weapons program was at the center of a 1999 espionage controversy involving lab scientist Wen Ho Lee. Though never charged with espionage, Lee was fired for security violations. He pleaded guilty to a felony count of mishandling classified information and admitted copying classified files. He said he disposed of them on site, but they were never found.

A year later, two computer hard drives containing nuclear secrets disappeared from a guarded vault at Los Alamos only to turn up behind a copy machine. The mystery has yet to be solved.

Last December, an inventory couldn't account for 10 computer disks, also used in the nuclear weapons program, prompting -- as was the case this week -- a brief suspension of classified work. Another disk was reported missing in May. Lab officials believe that in both cases the materials were destroyed with no records kept.

A classified floppy disk reported missing from Sandia National Laboratories in Albuquerque, N.M., was found Friday, but officials were tight-lipped about details surrounding that incident. The disk was listed as missing during a June 30 inventory.

The repeated security flaps as well as the scandal over fraudulent use of credit cards prompted the Energy Department last year to put its lab management contract up for bid when it expires in 2005, possibly ending the University of California's 61-year involvement.

"We have a huge number of exceptionally bright people here," Los Alamos spokesman Kevin Roark said Friday in a telephone interview. "But we still have what appears to be a small number of knuckleheads who ruin it for everybody."

But others suggest there are systemic problems at the heart of the lab's frequent flirtation with trouble.

And it doesn't always involved security.

The credit card fraud scandal in 2002 brought charges of an attempted cover-up after the lab fired two investigators it had assigned to get to the bottom of the case. One of them eventually received a nearly $1 million settlement with the university. Auditors found $4.9 million in questionable credit card expenses over four years, although lab officials said all but $195,246 had been accounted for.

Twice in four months last year two Los Alamos workers were contaminated from exposure to plutonium. The more recent case last August prompted a $770,000 fine from the Energy Department. But the fine will never have to be paid because by law the University of California, as a DOE contractor, is immune from such penalties.

Steve Aftergood, director of the Washington-based Federation of American Scientists' project on government secrecy, says a key question is whether the security flaps stem from sloppiness or willful disregard for the rules.

"Why would they do such things?" he wondered, noting that the lab is the workplace of some of the country's smartest scientists -- many of them long involved in highly classified defense work.

"These are brilliant scientists," added Danielle Brian, executive director of Project on Government Oversight, a watchdog group that has worked closely with whistle-blowers at Los Alamos. "They are told daily they are brilliant scientists. That creates a hubris ... almost a defiance. ... They believe the work they are doing is so important that it supersedes everything else."

In truth, problems are not new at Los Alamos. In 1945, Klaus Fuchs, a German-born physicist involved in the Manhattan Project, gave the Soviet Union the main elements of the design of the atomic bomb. He later admitted the espionage and was sentenced to 14 years in prison. He was released after nine years and went immediately to East Germany.