Nurse's conviction for four murders cast into doubt with new evidence
Colin Norris, from Glasgow, was convicted of murdering four elderly patients and trying to killer another in 2008.
The conviction of a killer nurse for murdering four patients has been cast into doubt after fresh scientific evidence emerged.
Colin Norris was jailed for life in March 2008 after he was found guilty of murdering the women while working in Leeds General Infirmary and the city's St James's Hospital in 2002.
New scientific evidence casts fresh doubt on the conviction of Norris, from Glasgow, who was jailed for a minimum of 30 years for his crimes.
A doctor raised the alarm after noticing that one of the patients had suddenly and unexpectedly slipped into a hypoglycaemic coma from which she later died.
A jury at Newcastle Crown Court was told that Ethel Hall, 86, who was not diabetic, had been injected with a massive and fatal dose of insulin.
The insulin reduced the sugar content in her blood to a level where her brain became starved of the glucose it needed to function properly. Tests showed insulin levels 12 times the norm, the court heard.
Norris has always protested his innocence and denied injecting patients with insulin. His case had been the focus of campaigners who fear a miscarriage of justice and is currently under review by the Criminal Cases Review Commission.
A BBC Scotland investigation has now raised the possibility that all of Norris's victims could have died from natural causes.
In a programme to be screened on Monday night, BBC Scotland Investigates: The Innocent Serial Killer?, Professor Terry Wilkin, an endocrinologist specialising in diabetes at the University of Exeter, questions the blood test on Ethel Hall.
Prof Wilkin suggests just over a litre of insulin would have been required to give the result used in court, a test result which was presented as proof of deliberate poisoning.
In the documentary, another expert, Dr Adel Ismail, a retired clinical biochemist, says another explanation for the blood result could be a rare condition called insulin auto immune syndrome (IAS).
According to the programme, IAS was said by prosecution experts at the trial to be too rare to be considered a possible explanation, but more cases have emerged since 2008.
Professor Wilkin said: "The data that has come from the analysis that was done on the samples that were given to the laboratory is perfectly consistent with insulin auto immune syndrome.
"So if you're asking me the question, does insulin auto immune syndrome fit with the facts of the case as reported, then yes it does."
Insulin poisoning expert Professor Vincent Marks told the programme it was wrong to conclude in the trial that hypoglycaemia is rare.
He said: "It wasn't as well known at the time of the trial as it is now that in the, particularly the elderly, frail, sick person, hypoglycaemia is far from rare." He concludes that the "verdict was unsafe".
The programme makers put the evidence they gathered to one of the jurors from Norris's trial at Newcastle Crown Court.
The juror tells the show: "If the new evidence was available at the time, I think they would have thrown the case out."
West Yorkshire Police said: "Norris was arrested, prosecuted and, on the basis of the evidence presented to the court, he was convicted and sentenced.
"His conviction was upheld at the Court of Appeal in December 2009. The case is currently under review by the Criminal Cases Review Commission (CCRC) and we will consider their findings when they are presented to us."
The BBC said it is making its evidence available to the CCRC.