Classified work at a key US nuclear weapons research lab has been suspended after sensitive data was reported missing.

The unprecedented stand-down at Los Alamos National Laboratory, New Mexico, began at noon yesterday after two important storage devices went unaccounted for during a 7 July inventory check. Officials refused to say what was on the Weapons Physics Directorate discs, citing national security concerns. The shut down will allow the intensification of a search, already into its eighth day.

"Until such time as we are confident that we are addressing this issue, then all activities with respect to classified materials have been put on hold," Gerald Parsky, chairman of the Regents of the University of California, which manages Los Alamos told a news conference yesterday. "These breaches of national security will not be tolerated."

The incident is the latest in a series of security shortcomings at Los Alamos - birthplace of the world's first atomic bomb - that have raised questions about the competence of its management. Keys to a sensitive area went missing for most of a day last month. In May, classified material was reported missing but managers later concluded it was intentionally destroyed.

Los Alamos has been under the microscope since November 2002, when allegations about purchasing fraud, equipment theft and mismanagement led to a review of the labs' business practices and the exit of some of its most senior managers. Their replacements are now under pressure.

The US government has opened up bids to manage Los Alamos after the University of California's contract expires next year. It's the first time in Los Alamos' 61-year history that this has happened and a sign of Federal discontent over how the facility is currently been run. ®

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