Five people died and 50 are injured following a terror attack near the Houses of Parliament on Wednesday.
The man shot dead by police after he ploughed a car into pedestrians on Westminster Bridge killing three people before storming the Parliamentary estate and stabbing Pc Keith Palmer to death has now been named as Khalid Masood.
Here's everything we know about the Westminster terrorist.
Who was he?
Burly body-building enthusiast Khalid Masood, 52, was born in Dartford in Kent on Christmas Day 1964. His mother Janet Ajao gave birth when she was just 17 years old and brought him up as Adrian Russell Ajao.
The married father-of-three spent years moving round the country - and the prison system - with a host of different identities, including Adrian Elms.
He was most recently based in the West Midlands and had a history of violent knife crime, converting to Islam after years of criminality.
Where was he from?
When Masood was two years old, his mother, now 69, married Philip and they brought him up in a £300,000 house in the seaside town of Rye, East Sussex.
Then, two decades later, they moved to Tunbridge Wells, Kent, where they lived with Masood’s half-brothers, Alex and Paul. Paul lives in Banbury, Oxfordshire where he runs florist and fabric companies.
Mrs Ajao now lives with Philip in the Welsh-speaking village of Trelech in rural Carmarthenshire where she sells textile goods that she sews.
Over the past five or six years, Masood, his wife, aged 39, and their young children, have been on the move. 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.
From Luton, Masood and his family moved to Forest Gate in east London where neighbours said he 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 they moved there, but that property was also raided by anti-terror police following the attack.
Did he have previous convictions?
Yes, he was known to police and had several previous convictions across 20 years, although he had never been convicted of any terrorism offences. His first conviction was in November 1983 for criminal damage when he was 18.
In a statement, the Metropolitan Police said: “He was known to police and has a range of previous convictions for assaults, including GBH, possession of offensive weapons and public order offences.”
What was the most serious?
In July 2000, aged 35, Masood snapped and slashed a man across the face with a knife after an argument that had "racial overtones". He was living in the quiet Sussex village of Northiam when in he attacked cafe owner Piers Mott in the face with a knife.
Hove Crown Court heard that the attack left Masood and his young family "ostracised" in the village. He had been in conflict with his victim before.
After leaving the pub, in which he argued with his victim, Masood lost his temper and slashed seat covers in Mott's car. When Mott arrived at his car, Masood waved the knife at him and caught his face, leaving him needing more than 20 stitches.
The court heard at the time that Masood had consumed four pints during the afternoon and had the knife because he was decorating his daughter's bedroom. Judge Charles Kemp sentenced Masood - then Elms - to two years in prison.
Three years later, and now out of jail, Masood was accused of stabbing a man in the nose in Eastbourne, leaving him needing cosmetic surgery. He was sent back to jail for another six months for possession of an offensive weapon.
What else do we know about him?
By 2005 he was working in Yanbu, Saudi Arabia, teaching workers at the General Authority of Civil Aviation (GACA) in Jeddah, according to The Sun, which says it obtained a copy of Masood's CV.
In the CV he is said to describe himself as “British”, “friendly and approachable” and a good listener and the document reportedly claims he had a economics degree. He apparently began in sales and was later promoted to manager at Aaron Chemicals in Bodiam, East Sussex.
In 2004 he is understood to have married a Muslim, Farzana Malik. It is unclear what became of their marriage and whether Masood converted to Islam at the time. In the same year, The Sun reports, Masood’s CV claims he gained a TESOL certificate, allowing him to teach English to foreigners.
In spring 2009, Masood reportedly returned from Saudi Arabia to the UK. After a five-month gap, he is said to have joined a TEFL college in Luton as a “senior English teacher”.
What do we know about his final movements?
On Wednesday, having stayed the night before the attack in a cheap hotel in Brighton, he got into a hired car, drove to London and then used it as a weapon to kill pedestrians on Westminster bridge.
Then brandishing a knife, he slaughtered Pc Keith Palmer who was trying to protect the palace of Westminster, in cold blood.
Masood told staff at the Preston Park Hotel: “I’m off to London today” as if he was a tourist. The capital, he declared, “isn’t like it used to be.”
Was he on MI5's radar?
He was known to the authorities and had come under the radar of MI5 some years ago as a "peripheral" figure in a terrorism investigation, Theresa May told MPs on Thursday.
However, at some point a decision was taken that he was no longer considered a threat and Mrs May said that at the time of Wednesday's attack he was “not the subject of any current investigations”.
She added: "There was no prior intelligence about his intent to mount a terrorist attack."
When was he radicalised?
No one knows exactly. Masood served time in Lewes jail, East Sussex, Wayland prison in Norfolk, and Ford open prison, West Sussex. It is quite likely he was radicalised during a spell in jail.
Did he have links to Isil?
The terrorist group Islamic State of Iraq and the Levant (Isil) claimed Masood was one of its "soldiers" in a statement released through Isil's Aamaq news agency on Thursday.
Isil has been explicitly threatening attacks on Britain for the last 18 months. At the end of a propaganda video claiming the Paris attacks in November 2015, Isil singled out the UK as its next target.
Did he act alone?
Police believe he acted alone, although they are investigating possible associates.
So far, ten people have been arrested and a number of addresses raided across London, Birmingham and elsewhere. Nine people remain in custody and one, a woman, has been released on bail.
Those arrested include a 39-year-old woman held at an address in east London the night after the attack on suspicion of preparation of terrorist acts.