Millions in Washington, Baltimore, Philadelphia and New York were at risk
The US Air Force almost detonated a huge atomic bomb over North Carolina in 1961, a newly declassified document has revealed.
Two hydrogen bombs were accidentally dropped over the city of Goldsboro just two days after President John F Kennedy's inauguration, when the B-52 plane carrying them broke up in mid-air.
And a report obtained by investigative journalist Eric Schlosser under freedom of information laws showed one of the devices began to detonate - but that a single switch prevented disaster.
Had the switch failed - as the other three safety mechanisms had - millions of lives across Washington, Baltimore, Philadelphia and New York City would have been at risk.
The bomb was 260 times more powerful than the one dropped on Hiroshima
US government scientist Parker F Jones' report was published for the first time in The Guardian newspaper, which said the bomb was 260 times more powerful than the one that devastated Hiroshima in 1945.
Mr Jones said in the report: "It would have been bad news in spades.
"The MK 39 Mod 2 bomb did not possess adequate safety for the airborne alert role in the B-52.
"One simple, dynamo-technology, low voltage switch stood between the United States and a major catastrophe."
The accident happened at the height of the Cold War between the United States and the Soviet Union.
While the US government has previously acknowledged the accident, the 1969 document is the first confirmation of how close the country came to nuclear catastrophe on that day.
Mr Schlosser said: "The US government has consistently tried to withhold information from the American people in order to prevent questions being asked about our nuclear weapons policy.
"We were told there was no possibility of these weapons accidentally detonating, yet here's one that very nearly did."
Mr Jones jokingly titled the report Goldsboro Revisited, Or: How I Learned To Mistrust The H-Bomb, a reference to Stanley Kubrick's classic 1964 film about nuclear armageddon, Dr Strangelove Or: How I Learned To Stop Worrying And Love The Bomb.
I often wonder how many Chernobyl's there have been and we don't know. Fluke this never "Seemingly" killing millions of people!!