Mr. Masood, born on December 25, 1964, in Kent, in the southeast of England, had recently lived in the West Midlands (center) and "was not under investigation" Scotland Yard.
He was known under different identities, police added on the man who launched his car on Wednesday against the crowd on the Westminster Bridge, facing Big Ben, killing two people - an American in his fifties And a British-born 43-year-old Spanish - and wounding several dozens.
The assailant then stabbed to death a policeman after he managed to enter the courtyard of Parliament, a symbol of British democracy, before being shot down.
"The author of the attack in front of the British Parliament in London is an EI soldier and the operation was conducted in response to the call to hit the countries of the" international anti-Jihadist coalition, "said Amaq, The propaganda agency of the IE. It is the first attack on British soil claimed by this extremist group which chained the military reverses in Iraq and Syria.
The British police, who believe that the man acted alone, however, arrested eight people at six different addresses in London, Birmingham and "elsewhere in the country."
Masood had been convicted on several occasions in the past for assault, possession of weapons and disturbing public order, said Scotland Yard. His last conviction was in December 2003 when he was arrested in possession of a knife
"A few years ago, it was investigated by the MI5" (Interior Intelligence Service) in connection with "violent extremism", explained Prime Minister Theresa May before Parliament, adding that he Was then "a peripheral character" in this investigation.
'We are not afraid'-
According to the Guardian, Masood was not on the MI5 list of the 3,000 most likely to commit a terrorist act.
On Thursday afternoon, reports from Belgium led to fears of an attack similar to that of London in northern Antwerp.
According to the police, a Frenchman, Mohammed R., was arrested Thursday after "putting a danger" of pedestrians by rolling at a very fast pace on a shopping artery of Antwerp, without causing injuries. Several weapons including a rifle were discovered in his vehicle, announced the Belgian federal prosecutor's office.
The attack on the Parliament of Westminster, committed one year to the day after the terrorist attacks in Brussels, killed 32 people, resembles those of Nice (France, 86 dead) and Berlin (12 deaths), both claimed in 2016 by the group Islamic state and already committed by launching a vehicle into the crowd.
Resolved to display its determination in the face of the most deadly attack in the United Kingdom over the last twelve years, the House of Commons, on the whole, observed a minute's silence before resuming its work on Thursday morning.
"We are not afraid," said Theresa May, speaking to the deputies, adding that democracy was "always to triumph".
The Prime Minister stressed that the attack did not call into question the official launch of the Brexit, scheduled for next Wednesday.
She paid tribute to the killed policeman, a "hero". A collection on the internet in favor of his family had collected more than 100,000 books Thursday afternoon.
- Candlelight Vigil -
US President Donald Trump tweeted that "a great American, Kurt Cochran, was killed in the London terrorist attack."
Twenty-nine people, including many tourists, were hospitalized. Seven are still in critical condition, the police said. Among the injured were three French students who were on a school trip.
French Foreign Minister Jean-Marc Ayrault met with their families on Thursday morning. He denounced an "attack on the heart of democracy" before going to Parliament.
It is the deadliest attack in the United Kingdom since the suicide bombings of July 7, 2005, claimed by al-Qaeda supporters, who had caused 56 deaths on public transport in London.
"London has already been there and knows how to take advantage," said Defense Minister Michael Fallon. "Londoners will not be intimidated by terrorism," said London Mayor Sadiq Khan.
A candlelight vigil was scheduled in the evening on Trafalgar Square in memory of the victims.
The perimeter around Westminster Palace, the political and tourist heart of the capital, was still closed on Thursday but the Westminster Bridge reopened.
"My thoughts and prayers and my deepest sympathy go to all those who have been affected by the horrific violence," said Queen Elizabeth II.
According to Scotland Yard, the British security services have thwarted 13 attempts at attacks since June 2013 in the United Kingdom where the level of anti-terrorist alert remains set at four on a scale of five.