A coroner said he was unable to conclude whether an SAS soldier killed rescuing soldiers in Iraq was shot by an insurgent or a fellow soldier.
Sgt Jonathan Hollingsworth, 35, was hit in the chest as his unit freed a group of Western security guards held hostage in a Basra apartment block in 2006.
The inquest heard he was killed with a bullet type used by British troops, but to which insurgents also had access.
Hereford coroner David Halpern returned a narrative verdict.
Such a verdict sets out the circumstances surrounding a death.
Hereford Coroner's Court had heard it was "well known" insurgents had access to British bullets.
Sgt Hollingsworth had been leading three teams on the operation on 24 November, 2006. He died in a military hospital.
Mr Halpern said it was known the soldier was killed by a gunshot wound.
"What we don't know is who discharged that bullet and where it came from," he added.
"Was it, sadly, own fire or was it insurgent fire?
"With those doubts in my mind, it does cause me some difficulties in looking at verdicts."
He said of the British soldiers who were with Sgt Hollingsworth in the apartment block: "It is difficult to comprehend that one of them would or could have accidentally discharged his weapon, but it is not beyond the realms of a possibility.
"We will say no further than that."
Returning the narrative verdict, Mr Halpern told the soldier's widow Kate: "I am very sorry that I have not been able to better answer some of the questions you must have."
Speaking after the inquest, Mrs Hollingsworth said she was "disappointed" but understood the coroner's decision.
'Bear no malice'
Her representative Andrew Davidson said: "She and her family only wanted to establish the truth about what happened to Jon, nothing more than that, and bear no malice to any individual.
"She is comforted to know that Jon was amongst friends when he died and they did their best for him.
"She recognises that they too have suffered loss."
He added that if there were "any lessons to be learned" the family hoped that they could be employed for the benefit of others.
He said that Mrs Hollingsworth was "grateful for the support from the Army and her friends at this distressing time".
A Ministry of Defence spokesman said: "Our thoughts and sympathies are with the family and friends of Sgt Jonathan Hollingsworth at this difficult time.
"Sgt Hollingsworth, a member of the Parachute Regiment, on secondment to Headquarters Multinational Division South East, Iraq, was killed on a successful operation to detain those who were known to attack both civilian and military personnel.
"He did not die in vain."