The US Air Force has relieved several officers of their commands after a B-52 bomber was mistakenly flown across the US loaded with nuclear-armed missiles.
Three colonels, a lieutenant colonel and 66 other personnel were punished following the incident at Minot Air Force Base, North Dakota, on 29 August.
Maj Gen Richard Newton said ground crews had failed to follow procedures.
The incident has been described as one of the worst known breaches of nuclear weapons procedures in decades.
Six cruise missiles armed with nuclear warheads were mounted on the bomber's wings before it was flown to Louisiana.
The missiles were supposed to have been taken to Barksdale Air Force Base, but the warheads should have been removed beforehand.
'Procedural errors'
Announcing the results of his six-week investigation, Gen Newton said there had been an "erosion of adherence to weapons-handing standards".
"They did not follow the formal scheduling processes that would have allowed them to do the proper maintenance and handling of those weapons"
"In the countless times our dedicated airmen have transferred weapons in our nation's arsenal, nothing like this has ever occurred," the Air Force deputy chief of staff for operations said.
Gen Newton said the "unprecedented string of procedural errors" had begun with a failure by airmen to conduct a required inspection of the missiles before they were loaded onto the wing of the B-52 at Minot.
The crew flying the plane were unaware it was carrying nuclear warheads, he said.
Experts have said that if the B-52 had crashed, there would not have been a nuclear explosion. However, there could have been a threat from plutonium leakage from the W80-1 warheads, which have a yield of five to 150 kilotons.
"This was an unacceptable mistake and a clear deviation from our exacting standards," Air Force Secretary Michael Wynne said.
"We hold ourselves accountable to the American people and want to ensure proper corrective action has been taken."
Both Mr Wynne and Gen Newton insisted the case was an isolated incident and that the current procedures for handling nuclear weapons were sound.
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