Police forces around the UK have been warned about possible problems with cells in their vehicles after a Co Derry man died exiting a moving PSNI van.
Paul Somerville, 21, managed to leave the "cell on wheels" while being taken from his home in Maghera, Co Derry, to Magheraberry Prison in January 2012.
He sustained serious head injuries. A doctor from a nearby health centre treated him at the scene before he was transferred to hospital, where he later died.
The cell door was misaligned with its frame and its latches did not always fully engage, forensic examination showed.
The incident was referred to the ombudsman.
Police Ombudsman for Northern Ireland Dr Michael Maguire, who investigated the incident, said the two PSNI officers accompanying Mr Somerville failed in their duty to ensure his safety.
The officers were disciplined by the PSNI but appealed, and sanctions were withdrawn.
Dr Maguire said an examination of the cell found the latches did not always fully engage, even when the door was slammed shut.
He also discovered that a deadlock did not engage unless the key was turned anti-clockwise through a full 90 degrees, even though a locking bolt could be seen moving as the key was turned.
Both officers involved in the case said they had seen the deadlock in the cell door engaging after the door was closed and one added she had pulled the door twice to check it was locked.
Dr Maguire said forensic examination showed that the door opened easily when pulled if it had not been properly secured.
Tests showed that even where the door's latches did not engage, it would not open if the deadlock had been fully locked.
One issue under consideration was whether the victim fell or jumped to his death.
Forensic evidence indicated that it would have taken a deliberate action by Mr Somerville to open the rear door because it could only be done by pulling a handle.
Two people spoken to by investigators said they had seen a man jumping from the van, but refused to provide formal statements, the ombudsman's office said.
Police had reported a suspected fault with the door when the van was serviced four days before the incident.
The office said: "The mechanic who did the service recalled that the door had been misaligned and said he had fixed the problem.
"However, the issue was not entered on the vehicle's records as it was not part of its normal service routine."
Dr Maguire recommended that cells and other modifications should form part of normal service routines.
His other recommendations have resulted in modifications by the PSNI.
Notices have been attached to cell doors warning officers to check locks are fully engaged, plates have been fitted to prevent doors from being opened from inside and larger viewing panels have been installed to improve monitoring of prisoners from the front of vans.
The man's parents, Desmond and Gwen Somerville, said the PSNI was not duty-bound to act upon the recommendations.
They asked: "When serious human error is involved, to whom can the general public go for justice?"
They added: "Our expectation was that while in police custody Paul would be safely conveyed. He should have been.
"Paul was our much-loved and only son, and we have been left devastated by his untimely death."