The US Air Force came extremely close to accidentally detonating an atom bomb over North Carolina, just three days after John F Kennedy's inauguration in 1961.
The hydrogen bomb explosion would have been 260 times more powerful than the nuclear weapons dropped on Japan.
The weapon was on board a bomber which broke up in mid air.
Details are revealed in a secret document published in a declassified form in the British newspaper, The Guardian.
Investigative journalist Eric Schlosser obtained the information through the Freedom of Information Act.
Two Mark 39 hydrogen bombs are said to have been accidentally dropped over Goldsboro, North Carolina, after a B-52 bomber broke up in midair.
The document says that three of four safety mechanisms designed to prevent unintended detonation failed on one of the bombs, and only one low-voltage switch prevented a cataclysm.
The US government has repeatedly denied its nuclear arsenal put Americans' lives at risk through safety flaws.
