Westminster killer Khalid Masood likely acted alone and it is possible the reasons why he carried out his attack will never be known, police have said.
Detectives are calling for anyone who may have spoken to Masood in the months leading up to his 82-second massacre, which claimed four lives, to come forward.
Dozens were injured when he ploughed his car into pedestrians on Westminster Bridge and outside the Houses of Parliament before being shot dead by police.
On Saturday the family of PC Keith Palmer, who intercepted Masood as he tried to storm Parliament, issued a statement thanking those who rushed to help the stricken officer in his final moments. “You did your best,” they said, “and we are just grateful he was not alone.”
Deputy Assistant Commissioner Neil Basu, of the Metropolitan Police, said: “We still believe that Masood acted alone on the day and there is no information or intelligence to suggest there are further attacks planned.
“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.
“We must all accept that there is a possibility we will never understand why he did this. That understanding may have died with him.
“Nevertheless, we are determined to understand if Masood was a lone actor inspired by terrorist propaganda or if others have encouraged, supported or directed him. If the latter proves to be the case, they will face justice.
“Whilst the attack lasted only 82 seconds it will remain in the memories of many forever. The thoughts of all those working on this investigation are with those so deeply affected.”
There is no evidence that anyone else accompanied Masood during the rampage in which he murdered four people and injured 50 others. But the timing and sequence of the radicalisation of Masood, born Adrian Elms, will, law agencies believe, provide important pointers towards what unfolded.
In pictures: Westminster attack
In pictures: Westminster attack
An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London
MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house.
Crowds gather in Westminster after shooting incident, which police are treating as terror attack
Police were also called to an incident on Westminster Bridge nearby
Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner
Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife
Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident
One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital
At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended
Masood had come to the notice of MI5 and Scotland Yard’s Special Branch in the past for associating with known extremists, but, security officials insisted, had not been involved in plotting terrorist acts.
It has emerged that Masood converted to Islam in prison.
The 52-year-old served several sentences for offences ranging from grievous bodily harm, to assault and possession of an offensive weapon dating back to his teenage years.
Violent episodes peppered his life, with one friend recalling Masood admitting he had dreams “about killing someone” 16 years before he murdered four people outside the Houses of Parliament.
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