The detective heading the Anne Grigg-Booth case said the former night matron was fully aware that she was poisoning her patients.

Det Supt Phil Sedgwick, of Keighley CID, said Grigg-Booth could have been on a power trip.

He said: "Diamorphine is a highly dangerous drug, where a patient can literally die on the point of a needle, and make no mistake she would have been fully aware of that fact. She administered lethal drugs to patients knowing she shouldn't have been doing it and knowing that patients weren't in pain."

He added: "It was never established why she carried out the acts. You could theorise about it but that's all it would ever be. Perhaps it was similar to Shipman and it was about power.

"Only she knew why. The upsetting thing for the families is that they now feel they may never know."

Det Supt Sedgwick said Grigg-Booth, who stood 6ft tall, always denied the charges and was not prepared to admit that she had broken the law. He said no-one would ever know how many patients she had killed. Despite investigating dozens of patient deaths, cremation meant there was no proof.

In the end Det Supt Sedgwick said the police and Criminal Prosecution Serv-ice (CPS) had to settle on only three murder charges.