The reason why nurse Lucy Letby went on a killing spree at a hospital neonatal unit may never be known, say detectives.
Prosecutors did not advance a motive as they outlined the allegations against her to the jury of eight women and four men at Manchester Crown Court.
Trial judge Mr Justice Goss told jurors they have did not have to be sure of the motive or motives for deliberately harming a baby.
He said: “Motives for criminal behaviour are sometimes complex and not always clear. You only have to make decisions on those matters that will enable you to say whether the defendant is guilty or not of the particular charge you are considering.
“Any decision you do make must be based on evidence and not speculation.”
Speaking after the case, senior investigating officer Detective Superintendent Paul Hughes said: “Ultimately, the only person who can answer the question ‘why?’ is Lucy Letby herself.
“I don’t think we’ll ever know unless she chooses to tell us.”
Deputy senior investigating officer Detective Chief Inspector Nicola Evans said: “The objective of our investigation was to provide the families and the parents of these babies the answers as to what’s happened to their child. And I do believe we’ve gone some way to do that.
“We haven’t answered all of those questions. Ultimately, one of the things that we haven’t answered is why.
“And that might be something that we never answer and, for Paul and I, that’s really hard to take, it’s really hard to take.”
Ms Evans went on: “I would describe Lucy Letby from my own experience of this investigation as beige.
“There isn’t anything outstanding or outrageous that we found about her as a person.
“And I think that has come across during the trial in that she was an average nurse, a normal 20-something.
“She had a healthy social life. She had a circle of friends. She had her parents and holidays. And there isn’t anything unusual in any of that and there isn’t anything that we have found that has been unusual.
“But clearly there was another side that nobody saw – a massive deceit – and that we have unravelled during this investigation and during the trial.”
Mr Hughes said fabricated or induced illness (FII) or Munchausen syndrome by proxy, in which a carer can physically harm a child, had been considered as an explanation.
He said: “Obviously she’s an attention seeker but I don’t know, it could be.
“We have intentionally not sourced psychological profiles of Lucy Letby because we have not received any evidence to do so. Therefore, we are not going to open a can of worms with that.
“There is no suggestion of mental illnesses or anything so we have tried not to go down the line of labelling it.”
He said Letby co-operated with detectives as she was interviewed for more than 21 hours across her three arrests.
Mr Hughes said: “She told us her version of events but she didn’t take it any further.
“She was very clinical without remorse or empathy.”
Ms Evans said she thought there had a been a lack of emotion from Letby in relation to the babies both in interviews and at the trial.
She said: “I don’t think anybody could sit and listen to the evidence that we’ve heard and not feel some level of sadness, and I don’t think we have experienced that from Lucy Letby or seen that from her during the trial.”
