More chilling details have emerged about a US atom bomb that nearly exploded in 1961 over North Carolina.
Declassified documents revealed that the US came perilously close to a world changing incident which would have left large parts of the country devastated for years.
The bomb would have been 260 times more powerful than the device that devastated Hiroshima, according to a declassified document.
The incident happened when two Mark 39 hydrogen bombs were accidentally dropped over Goldsboro, North Carolina, after a B-52 bomber broke up in midair.
And all that seemed to be between the atom bombs and disaster was a small low-voltage switch.
One of the two bombs carried by the B-52 bomber had been set to "armed" when it hit the ground.
The new report, released by the National Security Archive, said that if the switch had not been damaged during the crash it could have been disasterous.
A family sustained injuries when the unexplained explosion tore through their backyard. They were treated for minor injuries by a doctor at his house, before returning to their home to find a 15-metre crater.
They soon discovered it was an atomic bomb.
The Guardian newspaper reported in September last year z document, obtained by investigative journalist Eric Schlosser under the Freedom of Information Act, gave conclusive evidence that the United States came close to a disaster in January 1961.
There had been persistent speculation about how serious the incident was and the US government had repeatedly denied its nuclear arsenal put Americans' lives at risk through safety flaws.
Fallout could have spread over Washington, Baltimore, Philadelphia and even New York City, the paper said, threatening the lives of millions of people.
The details described in Schlosser's report were echoed in the newly released documents by Bill Burr of the National Security Archives.
“The report implied that because Weapon 2 landed in a free-fall, without the parachute operating, the timer did not initiate the bomb’s high voltage battery (“trajectory arming”), a step in the arming sequence,” Burr wrote.
“For Weapon 2, the Arm/Safe switch was in the “safe” position, yet it was virtually armed because the impact shock had rotated the indicator drum to the “armed” position. But the shock also damaged the switch contacts, which had to be intact for the weapon to detonate.”
“Perhaps this is what Secretary of Defence Robert McNamara had in mind, a few years later, when he observed that, ‘by the slightest margin of chance, literally the failure of two wires to cross, a nuclear explosion was averted.’”
Parker Jones, a senior engineer in the Sandia National Laboratories responsible for the mechanical safety of nuclear weapons, concluded in a document released last year that "one simple, dynamo-technology, low-voltage switch stood between the United States and a major catastrophe."
Jones' report, titled "Goldsboro Revisited or: How I Learned to Mistrust the H-Bomb," was written eight years after the accident in which one hydrogen bomb fell into a field near Faro, North Carolina, and the other into a meadow.
He found that three of four safety mechanisms designed to prevent unintended detonation failed to operate properly in the Faro bomb.
When the bomb hit the ground, a firing signal was sent to the nuclear core of the device and it was only the final, highly vulnerable switch that averted a disaster.
"The MK 39 Mod 2 bomb did not possess adequate safety for the airborne alert role in the B-52," Jones concluded.