SENIOR officers, including three colonels and a lieutenant-colonel, are among 70 personnel to be punished for slipshod practices that allowed a B52 bomber to fly across America carrying six nuclear-armed cruise missiles that should never have been loaded under its wings.

The incident triggered a rare “Bent Spear” alert – code for an incident involving live nuclear weapons - which raced to the secretary of defence and the White House, leaving red-faced air force commanders with a lot of explaining to do.

The nuclear weapons were “lost” for 36 hours after taking off on August 29 on a cross-country journey from the remote Minot air force base in North Dakota to Barksdale in Louisiana. Major-General Richard Newton, air force deputy chief of staff, said there was an “unprecedented” series of procedural errors, which revealed “an erosion of adherence to weapons-handling standards”.

It was the first time in the atomic age that the US air force had lost track of nuclear weapons. Any problem of “loose nukes” was thought to affect former Soviet rather than American weaponry.

The bungling began after a batch of advanced cruise missiles, developed in the 1980s with a “stealth” capability for evading Soviet radar, was scheduled to be destroyed at a munitions graveyard in Louisiana. Weapons handlers using an out-of-date identification schedule failed to notice that one bundle of cruise missiles in a hangar at the Minot base was carrying nuclear warheads before wheeling them on to the apron.

The W80 nuclear warheads, each with the destructive power of 10 Hiroshima bombs, should have been removed and replaced with a steel dummy, but ground crew loaded them on to a trailer without inspecting them. The driver called in their identification numbers to the munitions centre, but nobody bothered to check them.

The crew then loaded the missiles under the B52’s wings without looking at the little glass portholes which show whether a live or dummy warhead is being used. The plane’s navigator glanced under only one wing – not the one carrying the nuclear warheads – before clearing the plane for take-off.

The pilots did not know what they were carrying. After landing in Louisiana, they went off to lunch leaving the unsecured weapons on the runway for nine hours. Only when they were unloaded did airmen at Barksdale spot the error and raise the alarm.

“This was an unacceptable mistake and a clear deviation from our exacting standards,” Michael Wynne, the air force secretary, said on Friday at the conclusion of the force’s six-week investigation.

The American military has never confirmed the location or movement of any nuclear weapons before. The incident came to light because the information was leaked to Military News, a defence newspaper. In an echo of the film Dr Strangelove, the White House had been assured that this type of event “could never happen”. Had the plane crashed, there would not have been a nuclear explosion but plutonium could have leaked into the surrounding area.

The real threat was not from the weapons, but from the staggeringly lax security used to guard them. It appears that airmen had replaced the official procedures for handling the missiles with an “informal” schedule of their own, according to an official. By August 29 they had already dispatched 200 missiles to Louisiana and grown careless about the drill.

Although the air force insists the incident was an extraordinary one-off occurrence, it is not the first time that Minot and Barksdale bases have been criticised for sloppy procedures. Minot is located in a freezing, treeless and remote patch of North Dakota where boredom quickly sets in. A three-year tour of duty there is known as “three winters”.

Hans Kristensen of the Federation of American Scientists said standards for handling nuclear weapons had become shoddy: “Part of the reason is that after the end of the cold war and the disappearance of the Soviet nuclear threat, the nuclear career was not very sexy. It was not the way to go if you wanted to advance.”

Colonel Bruce Emig, who was in charge of the Minot base, has been relieved of his command, along with the munitions squadron commander. An air force spokesman said the incident could lead to courts martial.

Robert Gates, the defence secretary, said last week that while officials wanted to reduce the chance of such an incident recurring, it “would be silly to promise it won’t happen again”.

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