British police have raided at least six addresses in various parts of Britain and arrested as many as eight people, after a lone attacker launched an assault in the heart of the British capital.
The attack on Wednesday afternoon outside the Houses of Parliament killed four, including the assailant, and injured 40, with seven still in critical condition.
As intense investigations continued, police late yesterday named the assailant as Khalid Masood, 52. He was born in Kent and detectives believe he was most recently living in the West Midlands.
Prime Minister Theresa May told Parliament that he was British-born, was believed to have acted alone and was known to the intelligence services. He was once investigated a few years ago over "concerns about violent extremism", Mrs May told lawmakers.
"He was a peripheral figure... He is not part of the current intelligence picture," she said, adding that police are working on the assumption that he was inspired by Islamist ideology.
Yesterday, the Islamic State in Iraq and Syria (ISIS) terrorist group claimed responsibility for the attack through its Amaq news agency, calling the perpetrator "an Islamic State soldier".
The attack took place as world leaders met in Washington to discuss how to deal with ISIS; it also coincided with the first anniversary of the Brussels bombings, which were claimed by ISIS.
Pedestrians on Westminster Bridge were mowed down by a speeding SUV on Wednesday afternoon, which left a trail of bloodied bodies on the ground. A Romanian tourist fell into the Thames and was rescued with serious injuries.
The lone attacker crashed his car into a railing outside the Parliament compound, then tried to enter the grounds, stabbing a policeman to death before being shot.
The authorities immediately stepped up policing on the streets and around transport hubs, including airports, even as they urgently reviewed security arrangements around the Parliament area. The review will most likely focus on the Carriage Gates entrance, which was used by the attacker.
Parliament went into immediate lockdown after the attack, with lawmakers holed up in chambers for hours as part of security protocol.
Meanwhile, in what looked like a copycat ploy thwarted by Belgian police yesterday, a Frenchman armed with a rifle and bladed weapons unsuccessfully tried to ram his car into a crowd in the port city of Antwerp.
Last night, Londoners held a candelight vigil at Trafalgar Square in remembrance of the victims of the terror blitz - Spanish teacher Aysha Frade, 43; American tourist Kurt Cochran, 54; and slain policeman Keith Palmer, 48.
Condolences poured in from leaders around the world, vowing solidarity in the face of terror.
Both President Tony Tan Keng Yam and Prime Minister Lee Hsien Loong wrote to Queen Elizabeth II and Mrs May respectively. "Singapore strongly condemns this attack, and stands in solidarity with the United Kingdom against such acts that attempt to incite fear, create divisions and disrupt our lives," wrote Mr Lee, who is in Vietnam on a four-day official visit.
Londoners were determined to get on with their daily lives, even as the Union Jack flew at half-mast outside the Palace of Westminster yesterday. Lawmakers returned to the crime scene where a parliamentary session resumed in the morning. Tourist attractions in the vicinity such as the London Eye soon reopened for business, as did the Westminster Bridge crossing.
Inside the chamber hall, Mrs May struck a defiant tone in her address to MPs, who turned out in full force.
"Today we meet as normal - as generations have done before us, and as future generations will continue to do - to deliver a simple message: We are not afraid. And our resolve will never waver in the face of terrorism," she said.
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