Detectives disclosed they have identified a series of people who they believe survived having saline drips contaminated with insulin.
Seven death are now being investigated at Stepping Hill Hospital, Stockport, in what Greater Manchester Police say is the "most complex" inquiry they have ever carried out.
But the force is now facing a potentially damaging civil action after lack of evidence forced them to drop charges against the only suspect - a nurse accused of poisoning her patients.
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Rebecca Leighton, a staff nurse at Stepping Hill, who had been facing trial over three suspicious deaths on two wards, had the case against her dropped on Friday.