WASHINGTON- A B-52 bomber was mistakenly armed with six nuclear warheads and flown for more than three hours across several states last week, prompting an Air Force investigation and the firing of one commander, Pentagon officials said Wednesday.

Rep. Ike Skelton, chairman of the House Armed Services Committee, called the mishandling of the weapons “deeply disturbing” and said the committee would press the military for details. Rep. Edward J. Markey, a senior member of the Homeland Security committee, said it was “absolutely inexcusable.”

“Nothing like this has ever been reported before and we have been assured for decades that it was impossible,” said Markey, D-Mass., co-chair of the House task force on nonproliferation.

The plane was carrying Advanced Cruise Missiles from Minot Air Force Base, N.D, to Barksdale Air Force Base, La., on Aug. 30, said the officials, who spoke on condition of anonymity because of a Defense Department policy not to confirm information on nuclear weapons.

The missiles, which are being decommissioned, were mounted onto pylons on the bomber’s wings and it is unclear why the warheads had not been removed beforehand.

The Air Combat Command has ordered a command-wide stand down on Sept. 14 to review procedures, officials said. They said there was minimal risk to crews and the public because of safety features designed into the munitions.

In addition to the munitions squadron commander who was relieved of his duties, crews involved with the mistaken load – including ground crew workers _ have been temporarily decertified for handling munitions, one official said.

The investigation is expected to take several weeks.

The incident was first reported by Military Times newspaper group.

“There is no more serious issue than the security and proper handling of nuclear weapons,” Skelton said in a statement Wednesday. “The American people, our friends, and our potential adversaries must be confident that the highest standards are in place when it comes to our nuclear arsenal.”

Skelton, D-Mo., said his committee will pursue answers on the classified matter “to ensure that the Air Force and the Department of Defense address this particular incident and strengthen controls more generally.”