As the most prolific child serial killer in modern British history, Lucy Letby's case has left victims' families and the public with one major question - why did she do it?
The nurse maintained her innocence through her long-running trial but will now spend the rest of her life behind bars after being convicted of murdering seven babies and trying to kill six others while she worked on the neonatal unit at the Countess of Chester Hospital.
Sentencing her to a whole life order Mr Justice Goss said there was a "malevolence bordering on sadism" in her actions, and that she had "no remorse".
Prosecutors never advanced a motive as they outlined the allegations against her to the jury and speaking after her trial, detectives said the reason for Letby's killing spree may never be known.
Some experts have suggested that Letby may have been suffering from Munchausen syndrome by proxy - a disorder where a caregiver may harm someone in their care to get attention.
Watch: Who is Lucy Letby? The 'average' nurse who became Britain's most notorious child killer
While parallels have been drawn between Letby and serial killers Myra Hindley and Rose West, more appropriate comparisons might be to historic cases of killer nurses, such as Beverley Allitt from the UK and Charles Cullen in the US, they suggested.
Allitt, 54, targeted 13 victims during a 59-day spree which saw her kill four babies and poison nine others at Grantham and Kesteven Hospital, Lincolnshire, in 1991.
Cullen, 63, murdered dozens, possibly hundreds, of patients during a 16-year career spanning several medical centres in the American state of New Jersey.
Criminology expert Dr Dominic Willmott said Letby may have been motivated by a “pathological desire for attention and sympathy”, with her text messages showing she wanted to “garner sympathy” from colleagues after the children’s deaths.
The criminologist, a senior lecturer in criminology at Loughborough University who previously authored a paper on the Beverley Allitt case, said: "In our analysis of healthcare professionals who perpetrate violence against their patients, especially children, offending appeared to be motivated by a pathological desire for attention and sympathy emerging as a consequence of their involvement in the case.
"There was a complex interaction between this and a history of personality disorder diagnoses and characteristics, and were often found to be highly sadistic and narcissistic as described by those who knew them."
He said text messages released during the trial appear to indicate Letby's efforts to garner sympathy from her colleagues following the children’s deaths, along with other evidence that she had to be "repeatedly asked to focus on other patients around the time of the death of other babies and her passing on death notifications to family members seems to indicate her desire to be personally involved in the case, even when doing so was likely to raise suspicions about her involvement".
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Dr Marc Feldman, Clinical Professor of Psychiatry and Adjunct Professor of Psychology at the University of Alabama, Tuscaloosa, and an international expert in 'factitious disorders' including Munchausen Syndrome and Munchausen by Proxy, said his latest book, Dying to be Ill (2018), has a long section on serial murder by Munchausen by proxy perpetrators - who are usually women.
He told Yahoo News UK: "There have been well over 100 such perpetrators of healthcare professionals and paraprofessionals that have been reported around the world.
"I would begin by pointing out that MBP is not a mental illness per se; it is a form of abuse in which the perpetrators (usually mothers) victimise one or more of their children.
"Usually the goal is not to kill the child or even induce permanent physical damage. After all, doing so removes the person the mother has been manipulating to get the attention, sympathy, care, and concern that they may feel unable to get in any other way.
"They also commonly seek to 'outsmart' high status professionals like physicians—it makes them feel an illusory sense of control and mastery. And many of them lack a clear sense of self due to issues like borderline personality disorder."
He added: "The difference between these individual cases and serial murder cases in MBP is that, in the latter, if a child dies or suffers permanent injury, there is an unceasingly supply of new patients who can be abused and possibly killed by nurses or other healthcare professionals."
Such people may crave excitement or praise from those who witness them under pressure, Dr Feldman added, or might get kudos for carrying on despite the 'bad luck' of being on duty when so many children happen to fall ill or die.
"These are general statements because obviously Lucy Letby is not shedding light on her particular motives," he said. "In fact, she may be uncertain herself.
"Certainly she knew what she was doing, and had the goal of severe harming or killing children, but these perpetrators often lack insight into the 'why' of their own behaviour. In that sense, it can often be compulsive or even 'addictive'.
James Treadwell, professor in criminology at Staffordshire University, said answering 'how' Letby had committed the crimes was more important than 'why', so it could be prevented from happening again.
“It’s with the ‘how’ question you can prevent these things happening again. We had Allitt and Harold Shipman in medical situations, if you don’t answer the ‘how’ question, tragic history, terrible history, bereaved families could happen again.”
What is Munchausen syndrome?
Munchausen syndrome is a rare psychological and behavioural condition in which somebody fabricates or induces symptoms of illness in themselves.
It is named after German aristocrat Baron Munchausen who was famous for telling unbelievable tales about his exploits and past.
Munchausen syndrome by proxy - which it is suggested Letby may have been suffering from - is a variant of Munchausen syndrome where an individual fabricates or induces illness in a person under their care, typically a child.
The NHS website refers to this as Fabricated or induced illness (FII) and calls it "a rare form of child abuse".
"It happens when a parent or carer exaggerates or deliberately causes symptoms of illness in the child," the website says. "The parent or carer tries to convince doctors that the child is ill, or that their condition is worse than it really is.
"The parent or carer does not necessarily intend to deceive doctors, but their behaviour is likely to harm the child. For example the child may have unnecessary treatment or tests, be made to believe they're ill, or have their education disrupted."
What are the symptoms?
According to the NHS website, fabricated or induced illness (FII) covers a wide range of symptoms and behaviours involving parents or carers seeking healthcare for a child, from exaggerating or inventing symptoms, to deliberately making the child ill.
These can include someone:
persuading healthcare professionals that their child is ill when they're healthy
exaggerating or lying about a child's symptoms
manipulating test results to suggest the child is ill, for example, by putting glucose in urine samples to suggest the child has diabetes
deliberately inducing symptoms of illness, for example, by poisoning their child with unnecessary medicine or other substances
The NHS adds that cases where the parent or carer wrongly reports symptoms are "much more common" than cases where they induce illness in the child.