The police handling of the case of an American driver who fled the UK after a crash that left a nurse unable to walk was "not acceptable".

Isaac Calderon returned to the US after a collision in Herefordshire that left Elizabeth Donowho with two broken ankles and a fractured sternum. Ms Donowho complained that police had not done enough to keep the 22-year-old in the country.

At first West Mercia police's internal standards team disagreed, saying that there was never any reason to arrest him. Now, the Police and Crime Commissioner's office has concluded that the force should have arrested the suspect - a move which should have kept him in the UK.

Sky News has seen a letter written to Ms Donowho from Tony Morriss, deputy chief executive for the West Mercia police and crime commissioner, which states that he is "of the view that most reasonably minded people, if they knew the circumstances of this case, would find that the response of the force in this case was not acceptable". It says: "Mr Calderon was a foreign national suspected of committing a serious offence for which there was a likelihood of him receiving a significant sentence if convicted at court.

"I believe that serious consideration should have been given to the possibility of him leaving the UK to avoid appearing at court." The letter then acknowledges the "striking similarities" between the case and the crash that killed teenager Harry Dunn - the American driver Anne Sacoolas, a US spy, also left the country. Mr Morriss wrote: "I believe most investigators would have been guarded about the risk of the suspect leaving the country.

"I believe that Mr Calderon's detention would have been authorised by a custody sergeant had he been arrested in relation to this case… that would have enabled West Mercia Police to impose bail conditions including the seizure of his passport. I believe there was reason to arrest him." Mystery surrounds exactly who Mr Calderon was working for after Ms Donowho was told by police that he had been visiting the British special forces base (SAS) in Herefordshire when the crash happened nearby last summer.

She told Sky News that she was "devastated" when Mr Calderon, who is accused of causing injury by dangerous driving, failed to turn up to a hearing at Kidderminster magistrates court on 1 December. A warrant was issued for his arrest.

In court, Mr Calderon's occupation was given as "American soldier.