The UK cannot compel the US military to take part in investigations of 'friendly fire' deaths, the foreign secretary admitted today.
Margaret Beckett explained US personnel fall outside of UK jurisdiction, meaning coroners cannot compel them to attend an inquest or release information.
She was speaking after the Oxford coroner criticised the US' refusal to cooperate on the inquest of Lance Corporal Matty Hull, who was killed when US pilots fired at his vehicle in Iraq.
Today Andrew Walker, Oxfordshire assistant deputy coroner, found Lance Corporal Hull had been killed unlawfully.
"The attack on the convoy amounted to an assault,' Mr Walker said.
He continued: "It was unlawful because there was no lawful reason for it, and in that respect it was criminal.
"I don't think this was a case of honest mistake. There is no evidence the pilots were acting in self-defence.'
Mr Walker criticised the US for refusing to cooperate with the investigation, noting: "I believe that the full facts have not yet come to light."
Ms Beckett told BBC News 24 she was "disappointed" he had not been able to conduct a full inquest.
Nevertheless, the foreign secretary said she was "very grateful" for an otherwise "very vigorous and thorough investigation".
There will be future investigations where American evidence will be important in assisting the coroner to reach a verdict, Ms Beckett confirmed today.
The government has repeatedly requested the US military cooperate with inquests and has stated they are not criminal investigations. American participated would be "warmly welcomed", Ms Beckett added.
Hull was killed and four others wounded when an A-10 tank buster plane attacked the armoured vehicle convoy he was travelling in. The pilots appeared to confuse orange panels, used to identify British vehicles, for Iraqi rocket launches.
The pilots declined to cooperate with the inquest and the military initially refused to release cockpit video footage until it was leaked to a British tabloid.