Nurse Lucy Letby has been found guilty of murdering seven babies at the Countess of Chester Hospital in a rare case that has shocked the nation.
An independent inquiry has been set up to understand how Letby was able to carry out the killings and attempt six others before being reported to the police.
The reasons why Letby, a neonatal nurse, committed the murders may never be fully understood, although prosecutors and other experts told jurors during her trial of several possible motivations.
Here, The Independent takes a look at some of the main theories discussed in court.
To gain attention of doctor colleague she was ‘infatuated’ with
One motivation put forward by the prosecution was that Letby attacked and killed babies in her care to gain the sympathy of a doctor who she had become “infatuated” with.
It was alleged she wanted to make herself the centre of his attention and focus.
Throughout the trial, Letby showed no flicker of emotion until 16 February when the medic, who cannot be identified for legal reasons, confirmed his name after he swore on oath.
Letby and people in the public gallery could not see the married registrar because he had asked to give his evidence from behind a screen.
His voice prompted her to break down in tears as she abruptly left her seat and walked towards the exit door of the dock.
When the time came for her to enter the witness box, she said she loved the doctor as a “trusted friend” but was not in love with him romantically.
She denied claims by the prosecution that she was “infatuated” with the doctor.
She enjoyed ‘playing God’
Child P, one of the triplets Letby murdered, collapsed on 24 June 2016 and preparations were to move him to another hospital.
Shortly before the planned transfer, Letby is said to have told a colleague - the one she was accused of being infatuated with - "he’s not leaving here alive, is he?".
During Letby’s trial, prosecutor Nick Johnson KC said she made the comment because she "knew what was going to happen".
He said: "She was controlling things. She was enjoying what was going on and happily predicting something she knew was going to happen.
"She, in effect, was playing God."
Letby had earlier pumped air into Child P’s stomach as she fed him milk – just 13 minutes after murdering Child, one of his brothers.
Letby got a ‘thrill’ from the ‘grief and despair’ of parents
Letby acted unusually when the babies she killed or tried to murder suddenly declined, parents and other nurses on the ward where she worked said.
The parents of Child I, who died after repeated attacks by Letby, told police they remembered her “smiling and going on about how she was present at [Child I’s] first bath and how much she had loved it”.
Mr Johnson KC suggested to Letby that she was “getting a thrill out of what you were watching, the grief and despair, in that room”.
Letby denied the accusation.
The serial killer also searched Facebook for the families of her victims on the anniversaries of their deaths and would often look for a number of them within minutes of each other.
In one instance she carried out a search on Christmas Day. Letby, giving evidence, said she would search for all sorts of people – not just the parents of babies on the unit.
She found caring for less sick infants ‘boring’
Letby is said to have argued with a senior colleague when asked to work in an “outside nursery” where babies were treated in preparation for going home.
The unit was split into four rooms – intensive care in nursery one, high dependency care in nursery two and the “outside nurseries” of rooms three and four, the court was told.
Senior nurse Kathryn Percival-Calderbank told jurors that Letby was “unhappy” if she was allocated shifts in either room three or four.
She said: “She expressed that she was unhappy at being put in the outside nurseries.
“She said it was boring and she didn’t want to feed babies. She wanted to be in the intensive care”.
Mrs Percival-Calderbank, who qualified as a nurse in 1988, added: “If anything was going on within nursery one you would find she would migrate there, as we would all do to go and help. She would definitely end up in nursery one to assist.
“It was more that we were worried for Lucy’s mental heath because it can be upsetting, emotional and sometimes exhausting as well at the end of a shift, if you’re constantly put in that stressed situation all the time.
“Sometimes you’ve got to come out of that environment and be in an outside nursery.”
Letby was ‘not good enough to care for them’
Letby wrote “I AM EVIL I DID THIS” on a post-it note found by police at her home, in what was the closest the prosecution got to a confession.
She also wrote: “I don’t deserve to live. I killed them on purpose because I’m not good enough to care for them. I will never marry or have children. I will never know what it’s like to have a family.”
Letby told the court the notes, written after she was suspended from work pending an investigation, showed the ramblings of her mental anguish following the deaths of the babies in her care.
The nurse said the notes also contained protestation of innocence and they were never held up in court as concrete proof of her motive.