Flowers around a photo of police officer Keith Palmer, who was killed in the March 22 terror attack in Westminster | Justin Tallis/AFP via Getty Images Westminster attacker acted alone, say British police “We must all accept there is a possibility we will never understand why he did this.”

British police investigating an attack on Parliament that killed four people and injured 50 believe that the attacker acted alone and for motives that may never be known.

Khalid Masood, who was shot dead outside of parliament after using a car to mow down pedestrians on Westminster Bridge, was a convert to Islam who spent significant time in Saudi Arabia and had been identified as a potential jihadist.

But after four days of investigation, the Metropolitan police said they had not found any evidence to indicate that Masood was acting on orders from any group. They were also unable to establish why he had carried out his attack, despite Islamic State propaganda claiming that he was a "soldier" for the terrorist group.

"We still believe that Masood acted alone on the day and there is no information or intelligence to suggest there are further attacks planned," Metropolitan Police Commissioner Neil Basu said.

While police rounded up 11 people in the aftermath of the attack, nine have been released. A 58-year-old man in Birmingham remains in custody and a 32-year-old woman from Manchester was released on bail. Police are still looking into media reports that Masood might have sent a message on the instant messaging app Whatsapp before launching his attack, as such behavior may indicate he had the help of an accomplice.

So far the investigation had failed to turn up conclusive evidence. "We must all accept there is a possibility we will never understand why he did this. That understanding may have died within him," added Basu.

Investigators piecing together Masood's background describe an intelligent but angry man who became estranged from school friends in Kent shortly after leaving secondary school in the 1980s, and had a history of violent explosions often involving knives. Jailed for two years in 2000 for slashing a man's face in a pub row, he converted to Islam and gained qualification to teach English as a foreign language in Saudi Arabia, where he spent several years.

Jihadism experts told POLITICO that Masood's profile was different from most 'lone wolf' jihadists due to one main factor: his age, 52. In an analysis of attacks by attacks by Islamist radicals or far-right attackers in 31 countries, the Royal United Services Institute think tank said that the vast majority were carried out by people under the age of 40, with most jihadist attacks carried out by people aged under 25.

British investigators are continuing to delve into Masood's background to gain an understanding of his motives. "Even if he acted alone in the preparation, we need to establish with absolute clarity why he did these unspeakable acts to bring reassurance to Londoners and to provide answers and closure for the families of those killed and the victims and survivors of this atrocity," added Basu.

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