Police experts will study every aspect of the life of the 52-year-old who wreaked havoc at parliament. But his motives are a conundrum that may never be solved

Around Christmas, neighbours saw a van outside a small but comfortable home in the Winson Green neighbourhood of Birmingham. Khalid Masood, his partner and two children were moving out. Their new home was close by, but infinitely less salubrious. Their canalside terrace house had been swapped for a tiny bedsit above a restaurant on a busy road.

This and many other details will be picked over by analysts seeking to reconstitute the life of the man who killed four people with a 4x4 vehicle and knives before being shot dead outside the Houses of Parliament last week. The official aim will be to build up a comprehensive picture of the man responsible for the most lethal terrorist attack in Britain since 2005, in order to understand how he could have been stopped. Another aim will be simply to understand what turns someone into a terrorist killer.

“Clearly the main line of our investigation is what led him to be radicalised,” said Mark Rowley, Scotland Yard’s top counter-terrorism officer, on Friday.

Yet even such a seemingly banal objective is problematic. Many experts now question the whole concept of radicalisation. The FBI talks of “pathways to violence”, stressing that every individual’s journey to extremist murder is unique. There are fierce debates over the role of ideology, social circumstances, individual personality traits, and mental illness. Analysts prefer to talk of “risk factors” rather than “root causes”.

But some trends are evident. One is age. Masood, 52, was an outlier. Ten years ago, the average age of attackers in the west was around 29. Now it is nearer 25. In France almost 2,000 teenagers have been radicalised by Islamic State, officials say, with a 121% increase between 2015 and 2016.

One possible reason for this is demographic – large numbers of adult second-generation immigrants. Another is intensive social media use on now ubiquitous smartphones, and consequent exposure to propaganda. A third is the specific appeal of Isis, which offers adventure, camaraderie, cash rewards and even sexual opportunity in a way that contrasts dramatically with the asceticism of previous militant groups like al-Qaida.

This also helps explain why the proportion of Islamic militants with criminal backgrounds has risen too. Former Isis militants in Europe have described the attraction of the group as like that of a gang – yet one in which violence and misogyny become transformed into “resistance” and “redemption” for erring sinners. The group’s debased message, stripped of all but the most simplistic and distorted theological or political argument, appeals too to those who have neither the intellectual equipment nor inclination to debate, argue or learn.

Masood was convicted of a series of violent offences – the most recent in 2003. He appears to have converted to Islam around then. A disproportionately high number of militants involved in plots in the west have been converts. In the UK between 2001 and 2013, 12% of “homegrown jihadis” were converts, but less than 4% of the overall Muslim population were. In the US, the total in 2015 was 40%, against an overall level of 23%. Do converts have something to prove? Are they, with superficial cultural and theological knowledge, more vulnerable to extremist interpretations of key teachings and texts?

Then there is identity. Masood, born Adrian Russell Elms, grew up one of two black men in a village in Kent, son of a single teenage white mother. Much of his life appears to have been marked by racial tensions. This too may help explain the appeal of not just conversion to Islam, but of extremism.

Facebook Twitter Pinterest A Metropolitan police photo of Westminster attacker Khalid Masood. Photograph: Metropolitan Police/PA

As early as 2008, French security services were stressing “split identities” as a key factor behind extremism. Isis has made efforts to project its so-called caliphate as blind to colour, ethnicity and nationality – in deliberate opposition to the discrimination it claims exists in the west. Isis followers have focused social media activity on incidents of racial violence or injustice in the west. “I am not British or Indian but a Muslim,” said one British teacher, son of south Asian-born parents, shortly before his death fighting with Islamic militants in Syria in 2015.

Masood does not appear to have tried to leave the UK to fight for the cause at any stage. But from 2004 to 2009 he seems to have been in Saudi Arabia, teaching English. A trip to the Islamic world is another common feature of the background of extremist attackers. Most spend a few months studying, rather than teaching, Islam and Arabic. Many, probably including Masood, are simply steeped in the more rigorous, intolerant and puritanical strains of the faith which, with their limited previous knowledge of Islam, they often do not question. Such strands facilitate, though do not guarantee, a later commitment to more aggressive ideologies. Some are clearly exposed to violent tendencies in establishments linked to extremists.

One factor that has emerged from recent research and reporting of extremist violence is the role of peers, associates and the family. Terrorism is a social activity, not one undertaken by crazed loners. Research last year found that 45% of Islamic militant cases talked about their inspiration and possible actions with family and friends.

Brothers are often involved – in Paris in November 2015, and Boston in 2013. A married couple launched an attack together in California. Close friends team up – such as the pair of converts who killed off-duty soldier Lee Rigby in London four years ago. A 2009 FBI study found an average of three, and as many as 14, “bystanders” in every attack. These were people who knew what was likely to happen but failed to stop it. Most were relatives or friends. Police investigating the Westminster attack will be aiming to find such “bystanders”.

Finally there is the speed at which the “pathway to violence” is travelled. British officials have spoken of “fast turnaround” radicalisation. Many experts are dubious. But fast or slow, few of those who end up as extremists wake up one day and decide they want to be Isis killers. The process is gradual, if sometimes rapid. Didar Mohammed, a would-be suicide bomber in Kurdistan, told me of “waking up from a nightmare” when he failed to trip his bomb. A 19-year-old Belgian Isis recruit who at the last moment escaped travelling to Syria, described how she “was no longer herself”. Again and again, former militants describe how they barely imagined their eventual destination when their journey to violence began.

This is important. When on Monday or Tuesday Masood hired the vehicle he used for the attack, had he made up his mind days, or weeks or months before? Could the humiliation of having to give up a family home have been the final element that tipped him into action? What was he saying on WhatsApp, the messaging service, minutes before he launched his attack? And to whom?

It is human nature to try to construct stories, to simplify chaos into linear narratives which provide answers, solutions, explanations. This is what the search for the key to Masood’s radicalisation hopes to find. But we know that we do not experience our own lives in this way. Masood, for all his violent tendencies and criminality was, like most murderers, an ordinary man. We will have some answers to our questions. But anyone hoping to definitively resolve the puzzle of how a 52-year-old from Kent came to kill and be killed in the centre of our capital city will be disappointed.