William’s disappearance is a “once in a ­decade crime”, according to Dr Olav Nielssen, a psychiatrist from the University of NSW who has studied juvenile homicides. More than half the children who are murdered die from violent abuse. Around a quarter are killed by psychotic parents or guardians. Ten to fifteen per cent of deaths are the result of teenagers fighting. And then there are the drug-addled parents who give their children methadone to help them sleep.

Once, every decade or so, “a child is abducted by some sexual deviant, or thrill killer, or in some other bizarre circumstance,” Nielssen says. These murders are extremely rare.

If someone took William, it is unlikely to have been their first offence, says Dr Michael Diamond, a forensic psychiatrist and criminal profiler. “You don’t just do this sort of crime for the first time,” he says. “You would have to overcome other experiential things that confront you if you are going to do something of that magnitude. Is it somebody who has access to a child, and thought about it and not done it because their contact is too obvious, and so it has been

a rehearsed and lived-in fantasy without the terrible outcome?” Diamond asks.