The head of the US nuclear weapons programme has resigned following the discovery of security lapses at the Los Alamos national laboratory.
Linton Brooks, head of the National Nuclear Security Administration, was asked to offer his resignation to George Bush, the US president, after officials uncovered a series of management and security issues.
"These management and security issues can have serious implications for the security of the United States," Samuel Bodman, the US energy secretary, said in a statement announcing Brooks' departure.
"I have decided it is time for new leadership at the NNSA."
Bush is expected to name an acting director soon.
Massive responsibility
The NNSA, an energy department agency, is responsible for maintaining the US nuclear weapons stockpile and reducing the global threat posed by weapons of mass destruction.
It was established 2000 after the Wen Ho Lee espionage scandal at the Los Alamos lab in New Mexico, birthplace of the atomic bomb and the site of US nuclear weapons research.
A Taiwanese-American scientist, Lee was accused of stealing secrets about the US nuclear arsenal for China in December 1999.
The charges were later dropped but Lee pleaded guilty to improperly handling of sensitive data.
Security lapses
Brooks' departure followed two other incidents at the lab.
Last June, a security breach at Los Alamos reportedly led to the theft of files containing personal information on more than 1,000 workers.
The breach had gone unreported for months.
In October, a drug raid on a Los Alamos trailer park turned up secret data taken from the lab by a female employee.
The discovery raised concerns that the woman might have tried to sell the information to support a drug habit.