The killing of Lance Corporal Matty Hull in an American air strike in Iraq was a "criminal, unlawful act" by pilots who opened fire "with disregard for the rules of engagement", a coroner ruled yesterday.

Oxfordshire Assistant Deputy Coroner Andrew Walker also scathingly criticised the US authorities for their lack of co-operation which had caused so much "pain and suffering" to L/Cpl Hull's family. The failure to hand over important documents has meant that "the full facts have not yet come to light".

L/Cpl Hull, 25, died when his convoy came under attack from two A-10 "Tankbuster" warplanes despite the British vehicles showing orange markings to signify allied forces.

The coroner said "the attack on the convoy amounted to an assault. It was unlawful because there was no lawful reason for it and in that respect it was criminal. The pilot who opened fire did so with disregard to the rules of engagement and acting outside the protection of the law of armed conflict."

Mr Walker said the actions of American officials towards L/Cpl Hull's family had been inexcusable. "They, despite request after request have been, as this court has been, denied access to evidence that would provide the fullest explanation of the sequence of events that caused the tragic loss of L/Cpl Hull's life.

"I hope that in future, this does not remain their position and it is recognised that these decisions cause great harm to those who make them and and are a profound disservice to those who lost their lives in the service of their country and to the families left behind who simply want to know the truth."

On the penultimate day of the inquest at Oxford Old Assizes L/Cpl Hull's widow, Susan, had made a personal appeal to President George Bush for release of material from the Pentagon.

Yesterday she said the lack of American co-operation was "very disappointing". She added: "Of course I am fearful it could happen again but that's outside my hands. I think there have to be lessons learned from this. It's very clear that after such a death, evidence is required - full evidence right from the beginning, co-operation with boards of inquiry and investigations right from the beginning. If that doesn't happen in other circumstances such as this, this will all have been in vain."

Asked about the US and Britain supposedly standing "shoulder to shoulder" in Iraq, Mrs Hull continued: "They were all together serving the same purpose. But when it comes to following on from that and supporting each other in situations after that I think they [British soldiers] have been badly let down."

Mrs Hull added: "I hold Popov-36 [the codename for the US pilot] responsible for my husband's death because he fired. I hope that the pilots are at peace with themselves and that they can move on with their lives. I am sure they feel remorse for what they did."

She said: "Matt ... was probably one of the most professional soldiers you could meet, which is something we find quite difficult because he was taken from us in a manner that was far from professional."

Following the verdict the US Department of Defence issued a statement denying there was a cover-up and said that L/Cpl Hull's death was a "tragic accident".

A spokesman said: "The [US] investigation determined that the incident took place in a complex combat environment, the pilots followed applicable procedures and processes for engaging targets, believing they were engaging enemy targets, and that this was a tragic accident."

The coroner's statement "The pilot chose not to take steps to confirm the identity of the vehicles in the convoy - that he could easily have taken. The pilot who opened fire did so with disregard for the rules of engagement and acting outside the protection of the law of armed conflict.

"I'm satisfied, having given careful consideration to all the evidence that I have heard in this inquest, that this is a case where I can properly consider whether an unlawful action and manslaughter applies here. I find there was no lawful authority to fire on the convoy. The attack on the convoy amounted to an assault. It was unlawful because there was no lawful reason for it and in that respect it was criminal. The pilot who opened fire did so with disregard to the rules of engagement and acting outside the protection of the law of armed conflict.

"I don't think this was a case of a honest mistake. The pilot chose to interpret the orange panels (placed on coalition vehicles to identify them as friendly) as rockets without taking steps to identify the vehicles as friendly.

"The courage and bravery of L/Cpl Hull and of those in that convoy cannot be underestimated and follow the tradition within our armed forces that we are all justifiably proud of. The determination and courage of the members of L/Cpl Hull's family cannot go without mention either.

"I would like to offer my deepest sympathies to members of the family. I have no doubt of how much pain and suffering they have been put through during this inquisition process and to my mind that is inexcusable. They, despite request after request have been, as this court has been, denied access to evidence that would provide the fullest explanation of the sequence of events that led to caused the tragic loss of L/Cpl Hull's life."