Agency nurse Emma Webster was later jailed for three years for stealing the opiate painkiller pethidine to feed her addiction.

Top officers believe Norris could have seen how Webster successfully evaded discovery for months and reasoned he could get away with injecting insulin to kill patients which has fewer controls over its use and is often undetectable.

The two cases also raise serious questions about checks on staff at hospitals in Leeds and the safety of patients – although there is general acceptance there are no means which can ever completely prevent criminally-minded health workers from breaking the law.

Webster, now 34, a mother-of-three of Bramley, Leeds, stole the drug between August and December 2001 from the infirmary, obtaining work despite a previous conviction for stealing pethidine in Bradford and on occasions was even the senior nurse on duty. Norris joined the ward in October 2001.

She stole huge quantities of the drug and falsified medical records to cover it up. Norris later used the opiate diamorphine to make some of his victims drowsy before administering massive overdoses of insulin.

His unusual use of opiates on patients had already caused concern among senior nursing staff and he had been warned about it. Norris clearly had a fascination with drugs and had asked to go on a course to learn more about them.

Det Chief Supt Chris Gregg, who led the Norris investigation, said: "Is it a coincidence that there were two rogue nurses on the same ward even though they were not working together in any way?

"Did Norris spot that Emma Webster was able to behave criminally and get away with it for a period and that perhaps instigated his belief that he could get away with it?"

Hospital chiefs carried out inquiries into both cases and now claim they have some of the strictest medicine control policies anywhere in the country. A total of 77 recommendations were made concerning Webster's case.

The "most troubling" was a culture in which concerns about staff practice or performance were handled with a "quiet word" which meant formal records of bad practice or unusual behaviour were not recorded. If they had, it might have flagged up Norris's use of opiates on patients who were not in pain.

An inquiry into Norris made 71 recommendations for change.

A survey of insulin storage in 2003 recommended installing locks on all fridges containing the drug. But a later review found this had been ignored due to financial cutbacks which meant requisitions for locks were refused.

A repeat audit carried out last year concluded that it was now safely stored but still readily available to health staff who sometimes need it urgently.

Insulin has been used worldwide by medical staff to kill patients but is notoriously difficult to trace as it leaves the body quickly – it was only by chance that a blood sample was taken from Norris's last victim Ethel Hall which proved beyond doubt that she had been injected with a huge amount of insulin. Either she had been given the insulin in error or it had been given deliberately.

Her death and those of the other victims could also be explained by other causes and ward deaths happen daily.

Mr Gregg said he believed staff at the infirmary had done a "pretty good job" in raising concerns when they did.

"An insulin injection is notoriously difficult to pick up. It passes through the body very quickly making it untraceable. It has been described as the perfect homicide agent," he said.

The hospital inquiry found there was nothing irregular about Norris's appointment or his references.

As a newly qualified nurse, detailed monitoring of his progress was made in his first eight months but this ended in May 2002 – the month he carried out his first attack.

Hospital chiefs say they were asked by police not to carry out their own investigation into Norris to avoid compromising the main inquiry and he remained suspended on full pay.

But it was discovered that Norris had worked four extra shifts as a bank nurse at hospitals in Leeds when he had claimed to be either off sick or attending a training course.

It was not until May 2004 that Norris was dismissed on the grounds he had not been able to fulfil his contract for 18 months.

He appealed and a panel backed the decision but offered to re-employ him as a healthcare assistant, although he would remain suspended.

In April 2005 he was dismissed again on the grounds he was unable to practice as a nurse, it would be inappropriate to employ him and it was unlikely he would be able to work for the foreseeable future.

Norris again appealed and this only finally failed in December 2005 – three years after he was first arrested.

Family and friends had noticed she was getting slightly more frail and forgetful in previous months but until then she had remained active.

Her only son John had even joked with her at her 90th birthday a few months earlier that she would get a 100th birthday telegram from the Queen.

She was taken to ward 36 at Leeds General Infirmary and it was here she became nurse Colin Norris's first victim. She survived a huge overdose of insulin but remained very ill and died the following January.

Mr Wilby said it was several months later when police contacted him to say they were investigating her collapse.

He was now convinced something untoward had happened to his mother and the other patients.

He had hoped she would come to live with his family at their Oxfordshire home as she became more frail but her collapse blighted her final months. She needed intensive nursing and had never really recovered her faculties.

"It's that kind of thing which makes me, not angry, but sad. It's very difficult to come to terms with," he said.

"As a result of someone's actions her last few months were taken away from her.

"The difficult thing looking back is that we had my mother's funeral and you start grieving and you come to terms with it but because we've had the trial hanging over us since 2003, we've not been able to do that properly."

Born in Kirkstall, Leeds, two weeks before the sinking of the Titanic, Mrs Wilby married her husband Norman in 1943 at Burley Methodist Church after they had met at Price Tailors in Kirkstall, which later became United Drapers and John Colliers. Son John was born the following year.

She worked as a secretary there and and later at Leeds City Council and the probation service. She retired in 1975.

She lived in Rawdon for 60 years. Her husband was a Labour councillor for 30 years, at one time chairman of Aireborough Urban District Council.

"He was chairman in Coronation Year which meant they had to go round unveiling a lot of plaques and official duties like that," said Mr Wilby.

They were a close couple and his sudden death in 1988 hit her hard but she joined a bereavement group in Ilkley, a book club and was a member of the Humanist Society.

She liked to travel by bus for coffee at Bettys in Ilkley or go shopping in Leeds. Her family took her for trips to Bolton Abbey, where she enjoyed walking in the woods, or Harrogate.

She had recovered from a heart attack in 1993 and continued to live at home. She was becoming more forgetful and had taken to keeping a daily diary in case something slipped her mind.

Her son found her somewhat confused and frightened after surgery at the infirmary and noted there appeared to be a shortage of staff on the ward – several times she fell out of bed and when she pressed an alarm to go to the toilet it went unanswered. "We were concerned and I think at the time a bit angry at one or two of the things that happened to mum in hospital about her general care," he said.

Perhaps these extra demands were the reason Norris attacked her. Her son, who gave evidence in his trial, found him offhand and unhelpful.

"His whole attitude was to just turn his back and walk away when I was speaking to him. I misunderstood when he was talking about her going to Chapel Allerton thinking this was a nursing home which she would not want, rather than another hospital where she could recuperate, but he didn't bother to explain."

On the night of May 17, Norris gave her the powerful painkiller morphine – even though she had not complained of pain. She became drowsy and shortly before his nightshift ended he gave her a huge overdose of insulin.

Ninety minutes later she was found semi-conscious, suffering a sudden hypoglycaemic attack.

She did go to Chapel Allerton Hospital where her family found staff much more helpful.

Later she went to live in a nursing home in Ilkley before dying at Airedale Hospital, near Keighley, on January 30 2003 from an infection. Her son said: "I can look back on my father's life, what he achieved and what kind of man he was, with very pleasant memories and a feeling of pride, but with my mother, who was an equally decent person, it's been very difficult to start to think of her in those sort of terms – I can't get beyond the last few months of her life.

"She went downhill from being a woman who was starting to become forgetful but was
still fairly mobile and living with a reasonable amount of dignity to not being able to move.

"It feels as if I never really laid her to rest.

"The distressing part is that she could have died in peace; she could have had two or three more years or gone on to 100."

More on this story:

Nurse who hated old killed four women in hospital

Public inquiry must be held says son of Norris victim

I can't grieve properly for mother

She was convinced she wouldn't leave alive

Patient collapsed... just as the accused predicted

Hatred of elderly turned carer into new Shipman

Did another rogue nurse inspire plot to steal fatal drugs?