Nuclear warheads capable of unleashing the equivalent of 10 Hiroshima bombs were mistakenly flown across the United States by a bomber crew who thought they were dummies, and the terrifying security lapse was not discovered for almost 36 hours, it has been revealed.
The Pentagon is examining how so many vital checks and balances, painstakingly set out during the Cold War era, broke down to cause an incident that military personnel are calling one of the biggest mistakes in US Air Force history.
The flight last month was the first time in 40 years that nuclear bombs have been flown over US territory without specific authorisation from the top of the air force. Critics have argued that safety procedures have been disregarded as funds and expertise are diverted to new wars in Iraq and Afghanistan.
The B-52 took off from the remote Minot air force base in North Dakota with 12 cruise missiles that were being taken out of commission and scheduled for burial in Louisiana. The warheads on the decommissioned missiles should have been replaced with dummies of the same weight, but personnel failed to notice that six of the 12 were fully operational nuclear warheads.
The flight, on 30 August, was kept secret by the US Air Force, until news leaked on to military websites a week later. The Washington Post yesterday catalogued the full chain of errors and oversights and revealed that some of America's most powerful nuclear weapons were in effect out of supervision for almost 36 hours.
The bomber had sat on the tarmac at Minot overnight, with nothing but routine security patrols guarding its payload, and then for a further nine hours at the Barksdale base in Louisiana before the missiles were unloaded and a shocked transport crew recognised the error. The incident was deemed so serious that it was immediately reported to the Pentagon's nuclear planning headquarters and to the Defence Secretary, Robert Gates, as a so-called "Bent Spear" event. Only "Broken Arrow" events are more serious – they involve the loss, destruction or mistaken detonation of a nuclear weapon.
"Clearly this incident was unacceptable on many levels," said an Air Force spokesman, Lt-Col Edward Thomas. "Our response has been swift and focused, and it has really just begun. We will spend many months at the air staff and at our commands and bases ensuring that the root causes are addressed."
The chain of errors began in the camouflaged storage bunker in North Dakota, where nuclear warheads are supposed to be visually checked through a small window in the missile casing, or marked with a ribbon, or otherwise catalogued using serial numbers, barcodes and other markings. The B-52 crew is also required to examine the missiles, but only the side carrying the six dummy warheads was checked in this case, it is believed.
The air force insists that the public was never in danger and that even if the bomber had crashed, fail-safe mechanisms would have ensured that the bombs could not detonate. Anti-nuclear campaigners said that the dangerous fissile material inside the warheads could have been released into the atmosphere if the missiles had been damaged.
Two separate investigations are under way, including one set up in the past few days under retired general Larry Welch, who once commanded the strategic bomber fleet, charged with examining if there are widespread lapses in the way munitions are stored and transported around the US.
Scores of correspondents on military discussion boards have expressed their surprise and alarm, and warned that standards have slipped since the height of the Cold War.
One former B-52 commander wrote: "I'm not sure where to begin. I'm outraged and embarrassed! Back in 1979 we had to sign for nuclear weapons verifying serial numbers, the security folks posted two-man guards at the aircraft, the cops enforced two-man maintenance crews access to aircraft, etc. What the hell happened here?"
Linton Brooks, the man who oversaw billions of dollars in US aid to help Russia secure its nuclear stockpile, told The Washington Post that nuclear weapons handling had moved down the agenda.
"Where nuclear weapons have receded into the background is at the senior policy level, where there are other things people have to worry about," he said.
Mr Linton resigned in January as director of the National Nuclear Security Administration.