SANTA FE, N.M. (AP) – Officials with the contractor that runs Los Alamos National Laboratory sent top-secret data regarding nuclear weapons through open e-mail networks, the latest potentially dangerous security breach to come to light at the birthplace of the atomic bomb, two congressmen said.

The breach was investigated by the National Nuclear Security Administration, which rounded up laptop computers from Los Alamos National Security LLC’s board members and sanitized them.

But NNSA and lab officials who subsequently appeared before a congressional committee investigating security problems at the nuclear weapons lab never mentioned it, according to a letter the congressmen sent Energy Secretary Samuel Bodman.

Representatives John Dingell, chairman of the House Committee on Energy and Commerce, and Bart Stupak, who heads the panel’s oversight subcommittee, called that “unacceptable” and demanded an explanation. Both are Democrats from Michigan.

“This facility’s mind-bogglingly poor track record makes me repeat my question: What do we do at Los Alamos that we cannot do elsewhere?” Mr. Stupak said Thursday.

The northern New Mexico lab has been plagued by security lapses, from missing data storage devices to the Wen Ho Lee case to the discovery of classified data on a computer found during a drug bust at a former lab contract worker’s trailer.

The problems led the Department of Energy’s inspector general to describe security at Los Alamos as “seriously flawed” and prompted federal officials last year to put the lab’s management contract up for bid for the first time in decades.

LANS, which took over the lab’s operation, is made up of the lab’s former manager, the University of California; Bechtel Corp.; and two other companies.

The e-mail case, the latest to come to light, was reported to NNSA by a University of California official on Jan. 19, according to the congressmen.

“Apparently, open e-mail networks were used by several LANS officials to share classified information relating to the characteristics of nuclear material in nuclear weapons,” the congressmen said in the letter.

The breach occurred when a consultant to the LANS board, Harold Smith, sent an e-mail containing highly classified, non-encrypted nuclear weapons information to several board members, who forwarded it to other members, according to a Washington aide familiar with the investigation who asked not to be named because the information is sensitive.

The notice went out that there had been a breach, an official was pulled out of a White House meeting and told, and Lawrence Livermore National Laboratory flew a team across California and recovered the laptops within six hours, the aide said.

Lawmakers were assured no damage was caused, according to the aide. NNSA spokesman Bryan Wilkes said: “As a matter of federal law, we don’t confirm, deny or acknowledge allegations of security violations.”

“Any allegations of potential security violations at our sites is fully investigated,” Mr. Wilkes said. “If procedures are found to have been violated, then appropriate actions are taken.”