Mr Perry said the fighter pilots would not normally have tried to speak to the helicopters. Such contact would be handled by the Awacs reconnaissance aircraft overseeing the whole operation. The failure of the Awacs crew to identify the helicopters as their own may have been because they had briefly diverted from their flight plan and the Awacs had lost contact with them.
As flags on government buildings flew at half-mast across the country, the Pentagon said it is mystified about why the US pilots should have thought they were firing at Iraqi helicopters. 'I find it very difficult to understand,' Mr Perry said. 'The helicopters do not look very much like each other.'
The British Defence Minister, Malcolm Rifkind, who is on a visit to Washington, said the shooting down of the helicopters posed 'painful and difficult questions'. 'Given normal procedures that are applied, it was quite incomprehensible,' he said. 'It was not in the fog of war. It was indeed in broad daylight, so a lot of very painful and difficult questions need to be asked, and more important, they need to be answered.'
General John Shalikashvili, chairman of the Joint Chiefs of Staff, said yesterday that there had been more than 31,000 flights over northern Iraq since 1991. All had been successful and all were under the control of Awacs aircraft. Gen Shalikashvili said that on the day the accident took place, 34 fighters had carried out patrols over Kurdistan. The general added that to avoid another accident, changes had been made to the rules of engagement. The Pentagon later said that the US had suspended almost all its flights over northern Iraq except for those by Awacs.
A special investigations team - which includes a British representative - is on its way to Turkey but is not expected to report for several weeks. Investigators will look at film taken by the gun cameras on the fighters, listen to tapes of radio conversations and interview the pilots and crew aboard the Awacs.
Mr Perry would not say how procedures had been changed, but presumably they involve getting permission from a more senior officer before shooting down any hostile aircraft. He said yesterday he had already taken 'the hair-trigger off to a certain extent'.
Gen Shalikashvili showed pictures yesterday demonstrating that a US Blackhawk helicopter with external fuel tanks does resemble a Soviet Hind helicopter of the type flown by the Iraqis.
A side-effect of the accident and the deaths of 15 Americans on board may be to increase doubts in Washington about the ability of the air force to identify and attack targets in Bosnia. It also dents the air force's reputation which, after sinking low in the Vietnam war, rebounded as a result of the Iraqi defeat in the Gulf war which was largely attributed to air power.
Some Republican politicians were quick to take advantage of the loss of the helicopters to blame the Clinton administration for cutting the defence budget. However, there seems to be no evidence that the quality of US forces has deteriorated since the Gulf war. A more likely explanation is that the ability of the air force to identify targets was oversold at the time.