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It is thought an electrical spark started in the area holding oxygen supplies and other support systems. The fire spread quickly in the oxygen-filled atmosphere of the capsule, killing the crew within seconds.
The space crew, flight commander Gus Grissom, Edward White and Roger Chaffee, were taking part in a test run for the launch of the first Apollo mission.
Navy Lieutenant Commander Chaffee, aged 31, had never flown in space before. Air Force Lieutenant Colonel Grissom, 39, was the first American to make two flights. Air Force Lieutenant Colonel White, 35, made America's first space walk.
It is feared the disaster on launch pad 34 could delay America's plans to put a man on the Moon by as much as a year.
The three men were in the command module, mounted on the Saturn rocket as if ready for launch, but Saturn was not loaded with fuel.
At 1831 hours one of the astronauts was heard to say, "Fire, I smell fire."
Two seconds later, another astronaut, probably Lt Col White said, "Fire in the cockpit."
The fire spread through the cabin rapidly. The last communication from the crew was heard just 17 seconds later.
The pressurised atmosphere inside the capsule meant the astronauts would not have had time to open the hatch.
Under ideal conditions, the process takes about 90 seconds. It involves venting the cabin to relieve the interior pressure which helps hold the door closed.
It took technicians on the outside about five minutes after the fire had started to open the hatch.
There will be a full investigation into what caused the fire, but already questions are being asked about whether safety corners were cut in the race to be first to the Moon.
The astronauts knew there were risks involved. Lt Col Grissom became the second American in space in the Liberty Bell 7. On splashdown, the space capsule filled with water and sank and he almost drowned.
A few weeks before the launch pad tragedy, he wrote: "There will be risks, as there are in any experimental programme, and sooner or later, we're going to run head-on into the law of averages and lose somebody.
"I hope this never happens, and... perhaps it never will, but if it does, I hope the American people won't think it's too high a price to pay for our space programme."
The Apollo mission's maiden flight was due to blast off into space on 21 February.
President Lyndon Johnson paid tribute to the astronauts saying: "Three valiant young men have given their lives in the nation's service. We mourn this great loss and our hearts go out to their families."
The Apollo 1 tragedy led to a major investigation and criticism was levelled at Nasa for its complacency in underestimating the likelihood of fire.
A number of modifications were made to the Apollo spacecraft: the hatch was redesigned and made easier to open from the inside, the interior of the capsule was made more fireproof, the atmosphere was changed to a less flammable mixture of nitrogen and oxygen rather than just pure oxygen and the astronauts were given fireproof suits.
The space programme was delayed but not halted. On 25 May 1961 President John F Kennedy had committed Americans to landing a man on the Moon by the end of the decade.
Apollo 7 was successfully launched on 11 October 1968 for its maiden crewed voyage.
Less than a year later in July 1969, Apollo 11 landed Neil Armstrong on the Moon.
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