A US atom bomb 260 times more powerful than the device that devastated Hiroshima almost detonated over North Carolina days after President John F Kennedy's inauguration in 1961, a newly declassified document reveals.
Details of the incident were obtained by investigative journalist Eric Schlosser under the Freedom of Information Act.
The document was discovered by Mr Schlosser as he was researching for his new book on the nuclear arms race, Command and Control, The Guardian reports.
The incident occurred when two Mark 39 hydrogen bombs were accidentally dropped over Goldsboro, North Carolina, after a B-52 bomber broke up in midair. Each bomb carried with it the equivalent of 4 million tonnes of TNT explosives.
Although the incident had already been reported on, there has been persistent speculation about the seriousness of the incident and the US government has repeatedly denied its nuclear arsenal risked American civilian lives because of safety flaws.
But the newly published document shows one of the two bombs behaved exactly in the manner of a nuclear weapon in wartime, with its parachute opening and its trigger mechanisms engaged. Only one low-voltage switch prevented a cataclysm.
Parker F. Jones, senior engineer at the Sandia National Laboratories responsible for the mechanical safety of nuclear weapons, produced the report eight years after the event.
According to document, which Mr Jones named Goldsboro Revisited or: How I learned to Mistrust the H-Bomb, devastation caused by the fallout could have spread over Washington, Baltimore, Philadelphia and even New York City
Mr Jones concluded that "one simple, dynamo-technology, low-voltage switch stood between the United States and a major catastrophe."
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 switched that averted disaster.
"The MK 39 Mod 2 bomb did not possess adequate safety for the airborne alert role in the B-52," Jones concluded.