The man who killed three people and injured at least 40 in the terror attack on Westminster was British born and has previously been investigated by MI5, Theresa May said on Thursday.
Although the attacker, who was fatally shot by police, was known to the domestic security service, it is believed he acted alone and was “not part of the current intelligence picture”, the prime minister said.
“Some years ago, he was once investigated in relation to concerns about violent extremism. He was a peripheral figure,” Mrs May told parliament. “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.”
Her comments were part of a show of defiance as MPs and peers returned to Westminster the day after the attack, in which the attacker drove a car into pedestrians and fatally stabbed a police officer guarding the Houses of Parliament before being shot three times by police.
“We meet here, in the oldest of all parliaments, because we know that democracy, and the values it entails, will always prevail,” the prime minister said.
The Metropolitan Police on Thursday morning said they had searched six addresses overnight in Birmingham, London and other parts of the country as part of the investigation.
Mark Rowley, the Met’s acting deputy commissioner, added that 29 people were treated in hospital after the attack and seven people remained in a critical condition. Mrs May said that in addition to 12 Britons, the wounded included nationals from 10 foreign countries, including three French schoolchildren.
Mr Rowley said the three victims killed were police constable Keith Palmer, aged 48, a woman in her 40s and a man in his 50s.
This was a revision from Wednesday night, when he said that four people had been killed by the attacker.
Mrs May said the “working assumption” was that the attacker was “inspired by Islamist ideology”.
But Michael Fallon, the defence secretary, told the BBC that he appeared to be a “lone wolf” who used “things from daily life, a vehicle, a knife”, which he said made such an attack “much more difficult to forestall”.
Mrs May said police would be increasing their presence across the country, but urged the public not to “be cowed” by the threat posed from Islamist terrorism.
The attack will lead to a review of security at Westminster, particularly around the Carriage Gates entrance where the attacker entered the precincts before being shot dead by armed officers.
But MPs are determined not to turn the Palace of Westminster into a fortress and the principal message from parliamentarians is expected to be one of defiance and an insistence that Britain’s democracy will not be disrupted.
Following confirmation that PC Palmer was unarmed, questions have been raised over whether all police guarding Westminster should carry firearms in future. But Yvette Cooper, the Labour MP who chairs the Commons home affairs select committee, told Sky News those judgments should be made by the expert security staff.
“If his intention was to gain access to this building, we should be clear that he did not succeed,” Mrs May said. “The police heroically did their job. But as is routine, the police together with the House authorities are reviewing the security of the parliamentary estate.”
The attack was the largest-scale terrorist incident in the UK since suicide bombers killed 52 people in London in July 2005.
The attack paralysed the heart of British government as members of parliament and officials were held for hours in the House of Commons, while Mrs May was whisked away to safety by car.
It was the latest in a series of terror assaults in Europe and came exactly one year after suicide bombers attacked Brussels, targeting the airport and a metro station in the city centre. It also bore similarities to the attack in Nice last year, claimed by Isis, when a man drove a truck through a crowd of pedestrians, killing 84 people.
President Donald Trump also condemned the attack and called Mrs May to express US support, his spokesman Sean Spicer told reporters in Washington.
Angela Merkel, German chancellor, said the country stood “resolutely on Britain’s side in the fight against any form of terrorism”. Bernard Cazeneuve, the French prime minister, expressed “solidarity” with the UK.
The attack began in mid-afternoon when a car driven by the assailant careered northwards across Westminster Bridge towards Big Ben, knocking down pedestrians. The area is normally crowded with tourists.
The vehicle crashed into railings surrounding New Palace Yard, a main entrance to the Palace of Westminster, before the driver ran round and attempted to force his way through the gates.
The man, said to be middle-aged, attacked PC Palmer using two 7in knives, before he was shot dead.
The government recently revealed that UK security services had foiled 13 potential attacks in less than four years. Counter-terrorism units in Britain are running more than 500 investigations at any one time.