The explosives were to be delivered to the Balad airbase north of Baghdad in a routine 20-minute flight. The aircraft burst into flames and wreckage was scattered over a wide area.
The disintegration of the Hercules had led military officials to believe that there was a large-scale detonation on board. One theory was that a bomb was hidden on board, but it is believed that the blast may have been caused by accident.
The RAF C-130K transport exploded in a fireball 20 miles north of Baghdad, killing nine RAF special services officers and one soldier believed to be attached to the SAS. Defence experts told The Independent that wreckage at the crash site pointed to an accident on board the plane, or a bomb planted by insurgents, rather than an incoming missile.
A former defence minister said it was "highly unlikely" that insurgents could have breached tight security to smuggle a bomb on board, but it was not being ruled out. A suicide bomber recently penetrated security at a US base.
Insurgents would have needed a highly sophisticated missile to bring down the plane at 15,000 to 25,000 feet. Missiles that could bring down planes at that height could also target passenger planes, said the former minister.
Investigators are also looking into a theory that part of the airframe snapped, causing a mid-air explosion. The planes used by 47 Squadron of the RAF who ferry special forces are ageing, although they are regularly checked and upgraded.
A video by insurgents broadcast by the Arab television station al-Jazeera, purporting to show the plane being brought down by two Stinger missiles, was widely treated with caution, although a wrecked engine shown on the video matched the Rolls-Royce Allison T56 turbo-prop used to power the C-130K. A large reinforced cargo door was one of the few parts of the plane that survived the blast.
Downing Street hinted that it could take weeks to determine the cause of the crash, because of the threat at the site from Sunni insurgents and the marshy ground on which the wreckage is lying.
The Ministry of Defence revealed that the RAF Hercules had been flying at the "normal operational height" for the area north-west of Baghdad, which has been the focus for activity by heavily armed rebels. Under RAF rules, the C-130K would have reached at least 15,000ft when it exploded. Experts said such an altitude should have put the aircraft beyond the range of all but the latest computer-controlled rocket systems.
Video footage released by the "1920 Revolution Brigade" appeared to show ground-to-ground rockets incapable of reaching the C-130K.
Jim O'Halloran, editor of Jane's Land-Based Air Defence, said: "The missiles on that video did not kill that aircraft, and we do not know of any other sort of weapons systems in terrorist hands capable of that task. No pilot would have been flying at less than 15,000ft, especially in an environment like Iraq. All the rules say that you ascend as high as you can as quickly as you can, precisely to minimise the risk of a missile attack.
"Either there was a massive malfunction on board or there was a third party involved who managed to cause an internal explosion."
The Princess Royal made a private visit to the home base for the Hercules crews at RAF Lyneham in Wiltshire, to meet members of 47 Squadron. She is Honorary Air Commodore at the base.Reuse content