The most recent security breach at Los Alamos National Laboratory prompted the lab to halt all classified work Thursday while officials conduct a wall-to-wall inventory of sensitive data.

The stand-down began at noon, and the inventory of CDs, floppy disks and other data storage devices is expected to be completed within days, lab spokesman Kevin Roark said.

Last week, the lab reported that two items containing classified information were missing. The items were identified only as removable data storage devices.

The incident was the latest in a series of embarrassments that have prompted federal officials to put the Los Alamos management contract up for bid for the first time in the 61-year history of the lab that built the atomic bomb.

Lab officials are searching for the items and investigating how they disappeared.

Individuals who had access to the items are being allowed to enter their workplace under escort only, and work has been shut down in part of the unit involved, the Weapons Physics Directorate, while the investigation continues, lab officials said.

The National Nuclear Security Agency, the federal agency overseeing the labs, sent a team to Los Alamos this week to investigate the loss.

The University of California, which has operated Los Alamos from its beginnings during the World War II race to build the bomb, has not decided whether to compete for the contract when it expires next year.

But UC President Robert C. Dynes warned Thursday: "These types of incidents are unacceptable, and they really do have to come to an end."

Similarly classified material was reported missing in May. Lab officials later said they believe the material was destroyed, as intended, but that the paperwork was faulty.