One of America's largest nuclear weapons research laboratories has suspended its activities after secret information went missing.
Los Alamos has suffered other security lapses
Officials are not saying which data has disappeared from the Los Alamos National Laboratory in New Mexico, but it is thought to be highly sensitive.
The laboratory was temporarily closed four years ago as forest fires got dangerously close to it.
Several security breaches have hit the birthplace of the first atomic bomb.
Its closure comes on the anniversary of the first atomic bomb test in Alamogordo, New Mexico.
Its head, John Browne, resigned in January last year, following allegations of theft and fraud, including allegedly questionable purchases and the disappearance of computers and other equipment from the complex.
A few months later, the US National Nuclear Security Administration (NNSA) was instructed by the government to make "aggressive and far-reaching" changes to tighten security at all nuclear weapons laboratories in the US.
Security lapses at all three major US nuclear weapons labs - Sandia and Los Alamos in New Mexico and Lawrence Livermore in California - have included the loss of keys, laptops and even a van, as well as the two-year disappearance of two vials of plutonium oxide.
As yet there is no evidence of any deliberate act to steal the data at Los Alamos - probably CDs - which was reported missing last week from a unit known as the Weapons Physics Directorate.
One official is quoted as saying there is no evidence yet that the missing data has even left the facility, although that may be difficult to prove one way or another.
LOS ALAMOS LABORATORY
Isolated site chosen by J Robert Oppenheimer, 'father' of atomic bomb
Facility built first atomic bomb, nicknamed 'The Gadget' and tested on 16 July 1945
Produced 'Little Boy' and 'Fat Man' atomic bombs dropped on Hiroshima and Nagasaki, Aug 1945
Part of US Dept of Energy and managed by University of California
Involved in nuclear weapons development and defence, energy and environmental projects
However, the shutdown is a sign of how sensitive the data is and how seriously the matter is being taken, says the BBC's Pentagon correspondent, Nick Childs.
"These breaches of national security will not be
tolerated," Gerald Parsky, chairman of governors of the University of California which manages Los Alamos, told Reuters news agency.
Officials are currently conducting a detailed inventory of sensitive data at the lab, logging CDs and floppy discs.
That work is expected to take several days.
Staff who had access to the items in question are being allowed into the plant under escort only.
The NNSA, the federal agency which oversees the industry, has sent a team to Los Alamos to investigate the disappearance.
The incident is doubly embarrassing because it comes at a time when concerns about nuclear weapons proliferation are becoming more prominent and governments and other experts are calling for heightened safeguards to prevent the spread of nuclear weapons technology, our correspondent says.
It is also the latest in a series of embarrassments that have prompted federal officials to put the Los Alamos management contract up for bidding.
Los Alamos: Scientist Wen Ho Lee detained on spying charges in 1999 and later released - the lab's head John Browne resigned in 2003 amid allegations of theft and fraud
Lawrence Livermore: Three security managers suspended in June 2003 after loss of electronic key went unreported for six weeks
Sandia: Van stolen from a secure area and driven through a fence before being left in a car park - a classified computer went missing from the same area
Those include the disappearance of similar classified material two months ago.
The laboratory has also been tarnished by the controversy involving Wen Ho Lee - a Taiwan-born scientist who was kept in solitary confinement for nine months, after being accused of passing nuclear secrets to China.
He denied any wrongdoing, and pleaded guilty to a single charge after the government's case collapsed in September 2000.
Founded during World War II by a small group of scientists and military personnel who were seeking to develop atomic weapons, the Los Alamos laboratory now has more than 12,000 employees working on a site roughly 70sq km.
It is now involved in studies on modifying some warheads, but much of the activity there and at other labs is now focused on maintaining the safety of the US nuclear stockpile and working on other non-nuclear research.