Actually the main cause were design flaws with the square windows were all replaced with oval windows due to metal fatigue at the corners if the square windows.....
(Taken from a report regarding the breakup of the comets
He said the painstaking analysis of thousands of fragments of the Comet involved in the Elba crash had revealed that the damage was caused by a fault in the plane itself.
In what Sir Lionel called "one of the most remarkable pieces of scientific detective work ever done", a team led by Sir Arnold Hall, director of the Royal Aircraft Establishment at Farnborough, subjected models, full-size aircraft and replicas to the most elaborate and searching tests ever carried out on an airliner.
One fragment collected from the scene of the crash showed that a crack had developed due to metal fatigue near the radio direction finding aerial window, situated in the front of the cabin roof.
The investigators found that a small weakness such as this would quickly deteriorate under pressure, and would rapidly lead to a sudden and general break-up of the fuselage.
In tests on another Comet aircraft, Sir Lionel added, the investigators had found that up to 70% of the aircraft's ultimate stress under pressure was concentrated on the corners of the aircraft's windows.
