As long as a decade ago, Khalid Masood's "violent extremism" had caused security services sufficient alarm to put him under investigation.
But by the time Masood (52) went on the rampage on Wednesday, driving a car at 50mph along Westminster Bridge, murdering two innocent pedestrians before plunging his knife repeatedly into a police officer guarding the Houses of Parliament, he had dropped off their radar.
Masood, a father of three, was no longer considered a threat.
Theresa May, in a statement to MPs at 10.30am yesterday, was quick to absolve her security services of blame.
"His identity is known to the police and MI5 and, when operational considerations allow, he will be publicly identified," she said.
"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 - he was not part of the current intelligence picture. There was no prior intelligence of his intent or of the plot. Intensive investigations continue."
Five hours later, the Metropolitan Police made public his name in a short statement that detailed Masood's long history of violent crime. But none, they stressed, for terrorism offences.
Masood was born on Christmas Day in 1964 in Kent but, crucially, had used a number of different aliases - which police refused to release - in the run-up to the attack. It is thought the terrorist changed his name to Masood, possibly after being radicalised.
Police admitted last night that Masood was not his birth name, adding to mystery about why that was being withheld.
The force was clear there had been "no prior intelligence" of Masood's attack but admitted he had been "well known" to the police for a string of previous convictions.
Masood, a burly body-building enthusiast, received his first conviction in November 1983 for criminal damage when he was 18 and his last one in 2003 for possession of a knife. In a 20-year criminal career he also received convictions for causing grievous bodily harm, possession of offensive weapons and public order offences.
It is understood that Masood, also known as Khalid Chaudry, went to jail for his offences, where there is a good chance he was radicalised.
It is at that stage, it is thought, he came onto MI5's radar. A Whitehall source said he had been a person of interest but "peripheral" to a terror investigation.
The source declined to identify the terror cell.
Over the past five or six years, Masood, his wife, aged 39, and their young children, have been on the move. Electoral roll records show him living in areas notorious for pockets of Islamist extremism.
He lived for more than two years until 2013 in Luton where Anjem Choudary, an influential preacher now in jail for terror offences, had been a regular and often mob-handed visitor.
A former neighbour in Luton, Katie Garriques (48), a former headteacher, remembered a "polite, shy" and a "quite portly man" whom she often saw gardening at the front of the property and playing with his children.
When she saw the photograph of Masood having been shot in Westminster, she recognised him instantly. "I'm just saddened. I feel sick to be honest," she said.
From Luton, Masood and his family moved to Forest Gate in east London. A neighbour, who asked not to be named, said Masood frequented a mosque in nearby Leyton.
At some stage Masood's wife had moved to a new property on the site of the Olympic village. A property there was raided by police on Wednesday night.
In the past year, Masood and his family moved to Birmingham to a block of flats at Quayside in Winson Green. It is not clear why, but that property was also raided by anti-terror police following the attack.
Student Kaodi Campbell (25) confirmed the man in the picture was her neighbour.
"He was always polite and would say 'hello, hello' to me," she said.
"You could tell they were religious, his wife always wore traditional dress. I last saw them just over a month ago. They had three children.
"He had a job and you would see him leaving for work or taking his children to school. He always helped me, I would ask him to jump-start my car and he would always help."
On Monday or Tuesday, Masood turned up at the car hire company Enterprise at its Spring Hill depot in Birmingham and rented the Hyundai SUV used in Wednesday's attack.
He gave his profession as a teacher and, it is understood, his address as a rented flat close to Edgbaston and not far from the Enterprise offices. On Wednesday at 11pm, armed police stormed the upstairs flat at Hagley Road. More than a dozen officers armed with machine guns stormed the premises, making three arrests.
Scotland Yard said yesterday it had made eight arrests, seven in Birmingham and one in east London of a 39-year-old woman. A property in Brighton and another in south-east London were searched, prompting speculation Masood had stayed in one or both of the addresses before the assault on Westminster.
David Videcette, a former Scotland Yard counter-terrorism officer and security expert, said it was odd that Masood had committed the atrocity at the age of 52.
"His age is surprising as most terrorists are radicalised at a much younger age," said Mr Videcette. "It would be my assessment that he has probably had quite a troubled past, with involvement in drink or drugs leading him into criminality.
"Then at some point, possibly in the last decade, he has converted to Islam and changed his name.
"Then it appears that he has fallen under the malign influence of others who have encouraged or persuaded him to carry out this attack, possibly for money for his family."
Mr Videcette added: "What will be key in establishing is why he has suddenly and quite recently moved to Birmingham, having spent most of his life in the south-east.
"Pretty much every terror case I worked on had some Birmingham connection somewhere along the line. It is likely there were people there who were part of his close circle and the police will want to look at that aspect very closely indeed."
Meanwhile, London Mayor Sadiq Khan told the thousands of people who responded to his personal invitation to come together in solidarity and sympathy at the Trafalgar Square vigil in London yesterday, that "those evil and twisted individuals who try to destroy our shared way of life will never succeed and we condemn them".
In praise of the bravery of the emergency services responding to the attack, Mr Khan said: "When Londoners face adversity, we always pull together. We stand up for our values and we show the world we are the greatest city in the world."
Earlier, UK Home Secretary Amber Rudd defended the intelligence agencies, saying: "The fact he was known to them doesn't mean that somebody has 24-hour cover." In a defiant message to a packed Commons, Ms May said: "We will never waver in the face of terrorism."
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