Sudden, life-threatening 'collapses' had struck 11 children on Ward 4 of Grantham District Hospital, Lincolnshire, within two months in 1991; four were dead.
Lincolnshire police felt sure all had been attacked malevolently, but they conceded that an extraordinary and tragic chain of natural phenomena was a plausible explanation of the heart attacks, respiratory failures and convulsions.
Parents were disturbed that the police were reviving traumas to which they were becoming reconciled. Nobody had told them their children had been nursed by a killer. One of the two paediatricians in charge of Ward 4 still doubted that a criminal had been shadowing his rounds even when police were summoned on 30 April.
But in May 1991, almost a month after Grantham hospital had called in the police, results emerged from tests that had been conducted at Surrey University. Paul Crampton, one of the young patients who had collapsed, had high levels of insulin in his blood - not insulin made by the body, but extraneous insulin that could have got there only through a syringe.
Of 19 attacks under investigation, Detective Superintendent Stuart Clifton's team knew now that at least one was a criminal event. Meanwhile, another branch of the investigation had highlighted a single name.
Work rotas matched to sudden collapses in patients' conditions showed that only one nurse had been present whenever catastrophe struck the ward. Beverly Allitt was arrested on 21 May.
At her house, police found the ward allocation book, a record of who nursed whom - with what approved medication. She said she had taken the book to compile a diary. But detectives discovered that she never kept a diary.
It was one of many lies she told during two days of questioning. Confronted by male and female detectives she was confident and challenging. 'Usually when somebody lies, they pause before answering a question,' one officer said.
'She came right out with the answer, and dared us to prove otherwise. At that stage we couldn't. She knew more than us about Ward 4.' But a much sturdier scientific investigative apparatus was being assembled. The highly qualified team included: Vincent Marks, the insulin expert at Surrey; John Emery in Sheffield and David Fagan in Nottingham, both pathologists; and Sir David Hull, a paediatrician also in Nottingham.
Focusing on 12 children, police sent the laboratories X-rays and samples of blood, tissue and organs. Two detectives were assigned to each suspicious collapse. Interviewing hospital staff and relatives, they checked Allitt's story. By the end of June, she was proven a liar; the police put her at or close to every bedside.
The pathologists were increasingly sceptical of contemporary explanations of the causes of the four deaths; and then Professor Marks discovered that a nine-week-old baby, Becky Phillips, had been given a massive, fatal dose of insulin. A ward journal detailing the daily nursing regime was found with the pages chronicling the period of collapses cut out.
In November, the police, the Crown Prosecution Service and experts were scheduled to spend two days discussing whether there was enough evidence to charge Allitt. They decided within one day that there was: she was charged on 21 November on four counts of murder and eight of assault. Further charges were to follow.