How could anyone kill two innocent young girls?
The question has been asked repeatedly in the months since the bodies of Holly Wells and Jessica Chapman were discovered.
We now know beyond doubt the identity of their killer: Ian Huntley. But why did he do it, and how was he able to carry out such a dreadful crime?
Ian Huntley showed little emotion during the trial
Now he has been convicted, prison doctors may get an insight into the mind of a man who killed without remorse, and then put on an amazing act as he tried to throw police off his track.
One expert believes that Huntley's behaviour has all the hallmarks of a psychopath.
It is a word that makes us shudder, conjuring up an image of a wild-eyed monster. But in real life, psychopaths can seem remarkably normal.
They often possess charm and an ability to put people at ease. They are practiced liars.
What makes them so dangerous is their lack of conscience. They are focused only on their own satisfaction. They feel nothing for their victims - no compassion, no guilt, no remorse.
Dr Julian Boon, a forensic psychologist at the University of Leicester, has made a provisional assessment of Ian Huntley, based on what emerged at the Old Bailey.
"There is a strong suggestion here that we are dealing with a psychopath," he says.
"It is very difficult to believe this was not sexually motivated."
Those who knew Huntley say he had a fascination with schoolgirls. Before he met Maxine Carr, he had relationships with girls in their early teens.
Acting on impulse
In court, he denied that there was any sexual motive for inviting Holly and Jessica into his home. Yet it seems clear - as the Crown suggested - that the two ten-year-olds were a temptation he could not resist.
The girls were probably hoping to see Maxine Carr. Huntley, who was alone at the time, seized the opportunity.
"It is typical of psychopaths to act impulsively, if they think they can get away with it," says Dr Boon.
"The girls only had to walk past his house for him to have the opportunity to have his wicked way with them."
He believes the way in which Huntley attempted to dispose of the bodies is also highly significant.
"He had zero regard for the girls. They were just a vehicle for his self-gratification, says Dr Boon.
"Psychopaths treat other people as if they do not have an identity. Look at what Huntley did with the bodies of the girls, trying to burn them. It was done with callous self-interest.
"His reaction to their deaths was all about self-preservation. He put his own future before the girls'."
It was striking that during Huntley's cross-examination at the Old Bailey, he displayed little emotion as he described how the girls died, and how he cold-bloodedly disposed of their bodies.
Maxine Carr played no part in the actual murders. When the girls died, she was a hundred miles away in Grimsby, at the home of her mother.
But she did lie for Huntley, by supporting his story in interviews with the police and the media.
She later told detectives that Huntley was afraid he would be "fitted up" for the murders because he had previously been accused of rape, a charge that was eventually dropped.
Believing him to be innocent, Maxine Carr decided to support her partner. In interviews with the police, she said she could not accept that he killed the girls.
"Maxine Carr was acting under Huntley's influence," says Dr Boon.
"It would have been sheer manipulation."
Huntley invented a story that explained how the girls had left his house alive and well. In the days that followed, he appeared to be doing all he could to help find them.
He told reporters: "While there's no news, there's a glimmer of hope. I think that's what we're all clinging onto. It's just very upsetting to think I might be the last friendly face that these girls had to speak to before something's happened to them."
It was an extraordinary performance from the man who had killed the two girls.
"His participation in the search confirms my view that this is psychopathic behaviour," says Dr Boon.
"To behave in such a way, and to face the father of one of the girls as if trying to help, is totally self-serving."
There is also the question of how Ian Huntley managed to convince Soham Village College that he was fit person to employ as a caretaker.
They even asked him how he would deal with a pupil who made advances towards him.
"I have sympathy with the people on the interview panel," says Dr Boon.
"Psychopaths are so convincing. You cannot understate the way these people present themselves. They have a lifetime history of presenting the face that is required."
Ian Huntley felt no guilt for luring Holly and Jessica to their deaths. Everything he said and did during the days that followed reveals him to be a man without conscience.
He was cold, calculating and ruthless. His only regret was being caught; his only tears were for himself.