In May 2017, a letter arrived at Cheshire Police headquarters. It was from the chief executive of the Countess of Chester Hospital.
The hospital had seen an alarming increase in deaths of premature babies in its neonatal unit and there were concerns something sinister was afoot.
The chain of events that followed would lead to the conviction of nurse Lucy Letby, the most prolific child killer in modern Britain.
At its height Operation Hummingbird, as the police investigation was dubbed, involved nearly 70 officers and civilian staff, with detectives gathering 32,000 pages of evidence.
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