Anne Grigg-Booth, 52, is thought to have administered sometimes lethal overdoses of diamorphine and other painkillers to male and female patients who ranged from 18 to 96 years old.
The body of Ms Grigg-Booth, who was divorced and had one child, was found at her home by a neighbour on Monday morning. Police are not treating her death is suspicious and are awaiting the results of a post-mortem examination.
She was on bail charged with three counts of murder, one of attempted murder and 13 counts of administering noxious substances to 12 patients at Airedale Hospital near Keighley, West Yorkshire and was due to stand trial next April.
The detective in charge of the case admitted yesterday that Ms Grigg-Booth's motive and the number of her victims may now never be known. Asked if she could have killed many other patients, Detective Superintendent Phil Sedgewick said: "She could have. We just don't know. Now we'll never know for sure. It is a very unsatisfactory ending."
An inquiry has been launched into the case by local NHS officials.
Ms Grigg-Booth had worked as a nurse for 25 years when she was arrested in August 2003 after a police inquiry into deaths at the hospital.
Detectives had been called in by the NHS trust after a routine audit picked up anomalies in death rates.
Ms Grigg-Booth was a night nurse practitioner, a senior role which meant she was often in charge of wards and had access to drugs such as diamorphine.
In September last year, she was charged with the murder of Annie Midgley, 96, June Driver, 67, and Eva Blackburn, 75.
She was also charged with the attempted murder of Michael Parker, 42, and with 13 charges of administering noxious substances to 12 other patients.
The youngest victim is believed to be Lorraine Boddy, who was 18 when she was admitted to the hospital in 2000 suffering from an ovarian cyst and almost died after supposedly routine treatment.
Ms Grigg-Booth, who lived alone, strenuously denied the charges and never claimed to have administered the drugs for "mercy killing" motives, police said.
Neighbours said she had begun drinking heavily and neglecting her health since being charged. Her face had become ruddy and bloated.
She appeared in court last October, wept in the dock and later told reporters: "It's awful - I have been a nurse for 30 years. I don't want to speak about it." When she was charged, her neighbour Elizabeth Milner, said: "I was shocked because she's a lovely lady, very caring.
Colleagues described her as popular and well-respected but said she could be overbearing. One nurse said: "She was totally in control, but did have a bit of a god complex. She thought she could do little wrong."
Another said that while nurses were not allowed to administer opiate medication such as diamorphine unless it had been signed off by a doctor, in the middle of the night drugs were often given by nurses and authorised by medical staff later.
'She was never ill. She was a tough thing'
Eva Blackburn, 75-year-old widow
Eva Blackburn was a healthy 75-year-old widow who loved playing bingo and was rarely ill. But within 24 hours of being admitted to Airedale general hospital and under the care of Anne Grigg-Booth, she was dead. Last night, her 84-year-old sister, Amy Newman, expressed frustration at being denied the chance to see the nurse tried and convicted. Mrs Newman said: "I would have just liked to know why she did it. Now I will never find out." Mrs Blackburn was admitted to Airedale general in November 2001 after she fell ill at her sheltered housing complex in Haworth. Just a day later, Mrs Newman was told that her sister had died. She is still waiting to discover what caused her sister's death. She said: "The warden called an ambulance after tea on the Monday and she died early Tuesday. It all happened so fast I don't really know what was wrong with her." She added: "She had never really been ill before. She was a tough thing. The toughest of us all. She was very jolly. She loved her bingo." Mrs Newman did not suspect anything was amiss until the police called her in 2003 to inform her that they were investigating Grigg-Booth. Mrs Blackburn's death was one of a number of "anomalies" that had shown up in a routine audit at the hospital. In September 2004, the nurse was charged with the murder of Mrs Blackburn, along with those of Annie Midgley and June Driver.
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