They thought he had been the model suburban neighbour: keeping himself to himself, washing his car, mowing his lawn, even passing on a few footballing tips to the local kids.

But when police arrived in the early hours of this morning to search the quiet cul-de-sac in Winson Green, Birmingham, where Khalid Masood had been living until December, they learned something very different.

They looked again at the photo of the man on a stretcher, fatally shot by police, the perpetrator of the Westminster terror attack. It was him.

Their polite, helpful neighbour of about three years had driven a car at pedestrians on Westminster Bridge and stabbed to death a policeman guarding the Houses of Parliament. The 52-year-old father of young children was the terrorist responsible for the deadliest attack in Britain since 7 July bombings of 2005.

He had murdered three innocent people and inflicted critical injuries on seven others. A fourth victim, a 75-year-old man, died from his injuries in hospital this evening.

Isis, seeking to claim a share of responsibility for what may in fact have been the actions of a lone wolf, had hailed him as a “soldier of the Islamic State [Isis]”.

“It’s mad that it was him,” said Kaodi Campbell, almost dazed as she watched the gathering news crews point their cameras at the house across the street from her home in the Winson Green cul-de-sac.

Woman who knew Westminster terror attacker 'shocked' to find out it was Masood

But it was him, and now they and Britain’s security forces will have to piece together why.

For the neighbours at least, it was baffling.

One local boy recalled being invited to join Masood and his son in his front garden for a football kickabout one summer holiday afternoon. The supposedly friendly father had passed on a few tips too, “on passing and touches”.

“He washed his car, mowed his lawn,” said Ciaran Molloy, 27, a mailing machine engineer who lived overlooking Masood’s new-build house. “He was quite friendly, polite in every interaction.”

In pictures: Westminster attack Show all 9 1 /9 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 PA wire In pictures: Westminster attack 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. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack 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 Reuters In pictures: Westminster attack 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 PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack 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 Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

A keen bodybuilder, he was more often seen in tracksuits than Islamic dress. His “missus” – reportedly his wife – would wear Islamic dress, without covering her face.

There were three children too – although, because the family “kept themselves to themselves”, numbers, ages and sexes seemed to vary with every neighbour you spoke to. Some children were old enough to be seen in school uniform heading to the local primary. The youngest was thought to be about three.

“He was a friendly, normal guy,” said Mr Molloy.

If the neighbours’ astonishment was entirely understandable, the extent to which the security services should have been able to see behind the façade will now, almost inevitably – and however unfairly – become the subject of intense debate.

Despite the impression he gave his more recent neighbours, Masood had form, for both petty criminality, and at least peripherally, for involvement in Islamist extremism.

Born Adrian Elms in Dartford, Kent, he was reportedly brought up in Rye, East Sussex by a single parent.

He is understood to have been jailed on at least two occasions, with reports on Thursday night suggesting he may have been radicalised behind bars.

One of those spells in prison is reported to have resulted from an incident when Masood stabbed a 22-year-old in the face in Eastbourne in 2003.

Theresa May herself made clear in the House of Commons – sitting again today in defiant refusal to be cowed by Masood’s killing – that his identity was known to the police and MI5.

“What I can confirm is that the man was British-born and that – some years ago – he was once investigated in relation to concerns about violent extremism. He was a peripheral figure.”

“The case is historic,” she insisted. “He was not part of the current intelligence picture.

“There was no prior intelligence of his intent – or of the plot.”

She added: “Intensive investigations continue.”

Westminster attack: What we know so far

A massive, fast-moving operation by the Met’s Counter Terrorism Command, involving hundreds of detectives has so far resulted in raids on three locations in Birmingham, as well as two addresses in London, and properties Carmarthenshire in Wales, and Brighton in East Sussex.

There have been eight arrests, seven made on Wednesday night, within hours of Masood’s attack.

Scotland Yard said a 39-year-old woman was arrested in east London on suspicion of preparation of terrorist acts. In Birmingham six people were arrested on suspicion of preparation of terrorist acts: two women aged 21 and 26, and four men aged 23, 26, 27 and 28. Another man, aged 58, was arrested this morning in Birmingham.

And – while appealing for more information about Masood – the Metropolitan Police has revealed a little of his criminal past. Born on Christmas Day 1964 in Kent, he was 18 when he received his first criminal conviction, in November 1983, for criminal damage.

Known by a number of aliases, he clocked up convictions for assaults, including GBH, possession of offensive weapons and public order offences.

But he had apparently been going straight for 14 years. His last conviction was in December 2003, when he was 39, for possession of a knife.

It is possible, though, that when the security services examine again the man who was only noticed on the periphery of extremism, they may find a trajectory becoming increasingly common among the ranks of Europe’s petty criminals turned jihadis.

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At 52, he may have been older than the average home-grown terrorist, but sources have indicated that – like many others – he may have been radicalised by extremists he met in prison.

Some neighbourhoods in the places where Masood is suspected to have lived – Luton and east London as well as Birmingham – have also acquired unenviable reputations for housing a small, but occasionally murderous Islamist extremist minority.

It is believed he regularly attended a mosque in Birmingham and was a Muslim convert. Security sources, however, refused to discuss details or the role the mosque may or may not have played in his radicalisation.

That, they said, was part of ongoing investigations.

It is also believed Masood had made trips to London from West Midlands to plan his attack.

On Tuesday, though, the day before the attack, Masood was back to the façade of respectable suburbanite.

Hiring the 4x4 Hyundai from the rental firm Enterprise in Spring Hill, Birmingham, he told them he was an English teacher – a claim that has yet to be verified, but may be unlikely given his criminal convictions.

Masood also told the car hire firm something else – he called the company to say he was likely to cancel the hire.

Perhaps one day we will know whether this was just another deceit, or whether, at the back of his mind, a middle-aged father was having second thoughts.