The question of who injected a lethal amount of morphine into the stomach of a dying pensioner in his Oxford hospital bed remained a mystery after an inquest yesterday.

Assistant deputy coroner Dr Richard Whittington said after a hearing in Oxford that on the balance of probabilities a diamorphine overdose had caused the death of 85-year-old William Rolfe, of Kings Close, Letcombe Regis, near Wantage.

However, forensic scientist Dr Nicholas Hunt said he could not rule out the possibility of natural causes being responsible for Mr Rolfe's death. Dr Whittington recorded an open verdict.

A murder investigation into Mr Rolfe's death is continuing six years after the retired car worker died at the John Radcliffe Hospital, in Headington.

Oxford Coroner's Court heard Mr Rolfe was admitted to the hospital on December 8, 2000, with abdominal pains. Doctors later told his family he was dying. Relatives consented to no resuscitation attempts being made.

On December 20, he was attached to a machine called a syringe driver, which slowly administered morphine painkiller to his stomach over a 24-hour period.

Two hours after the third 24-hour treatment began, Mr Rolfe died. Nurses found the syringe was fully depressed. It had been removed from its protective cover and replaced the wrong way round and was found out of place.

The inquest heard that the only people with Mr Rolfe in the hour before his death were his wife of 61 years Gladys, son Michael, of Cane Lane, Grove, and his partner Susan Belcher. Each was arrested on suspicion of murder, but no charges were laid.

At the inquest, nursing staff said, prior to his father's death, that Michael Rolfe had told them words to the effect of: "If this had been one of my dogs he would have been taken out and shot."

Detective Superintendent George Turner said that in an interview, Mr Rolfe denied he made such remarks.

Dr Whittington said in his summing-up: "Said to doctors and nurses early in their careers, these are fairly shocking words. But when death is inevitable, it's understandable that someone might speak in such a manner."

Nurses said that Michael Rolfe removed an oxygen mask from his father, but replaced it after doctors told him it was there to ease Mr Rolfe's pain.

Dr Whittington absolved staff at the hospital from any blame and said tests carried out on the syringe driver proved it was not faulty.

Dr Hunt said that, on the balance of probabilities, the cause of death was unnatural, but he could not rule out the possibility of heart failure, due to severe valve disease, as the cause.

After the inquest, Mr Turner said the investigation wasn't closed and asked for anyone with information to call Thames Valley Police on 0845 8505505. He added: "It's a long time ago, but hopefully this will jog the public's memory."