As the smoke cleared from the ground-floor flat in southeast London last Sunday, the fire brigade discovered a scene of carnage. Yet the Metropolitan Police did not call a press conference to report on the murders of Laurent Bonomo and Gabriel Ferez until four days later. The delay is a sure indication of the challenge and complexity that the investigation faces. The culprit in most murders is established in the first few hours. In those cases there is usually little need to seek assistance from the public.

The challenges increase as the details unfold. The media interest that will swirl around the killings will increase the challenge of getting to the heart of how such a horror could occur. The reported hundreds of stab wounds, binding and fire-setting also test the investigators to make sense of what went on. For although the fire-setting suggests preparation and criminal intent, why would criminals bind and torture their victims before killing them?

As career criminals become ever more aware of the power of forensic science, arson is becoming the favoured method to destroy DNA and other evidence; sometimes known as the “ CSI effect”. But this determination to get rid of the evidence so totally actually reveals as much as it was intended to hide. It points to people whose DNA may already be in national databases. Why would someone who had brought an accelerant stab the victims so often and so mercilessly? The ferocity of the killings is not typical of experienced criminals who want to get away as soon as they can and may well have access to weapons that allow quick deaths. It points more to a confused mixture of habitual criminality and disinhibited anger. One further challenge the police must face is to avoid being mesmerised by the horror of these killings. So much murder is terrifyingly banal.

A further complication is how two healthy, intelligent young men were controlled and bound. More than one offender seems likely. Most people would run out of steam after stabbing someone 10 or 20 times. More than 200 stabbings indicate an extended process that could be explained by two or more people goading each other on, aggravated by drugs or alcohol. That adds further challenges to the investigation in trying to establish how many culprits there might have been and what were their relationships to each other.

The police must have been hoping for a quick solution to such an unusual double murder, but failing that, they could not hold it from public scrutiny any longer and must hope that their publication of the details will light up the trail to these vicious killers.

David Canter is Professor of Psychology at the University of Liverpool.

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