British police raided several properties and arrested seven people in connection with the attack outside Parliament that left three dead when an a knife-wielding man mowed down pedestrians on London's Westminster Bridge and fatally stabbed an officer, a senior police official said Thursday.
Some 40 people were wounded in what Prime Minister Theresa May condemned as a "sick and depraved terrorist attack." The attacker was shot dead at the scene.
Metropolitan Police counterterrorism chief Mark Rowley said that he believed the attacker acted alone and was "inspired by international terrorism" and Islamic extremism.
The threat level for international terrorism in the UK was already listed at severe, meaning an attack was "highly likely."
Police raided six addresses, including some in the central city of Birmingham, and arrested seven people in connection with Wednesday's attack, Rowley said.
The attacker has been identified and was known to British security, according to a British security official. He declined to name the man and to give any other details about his identity, nationality or hometown.
He revised the death toll from five to four, including the attacker, the police officer and two civilians. He said that 29 people required hospitalization and seven of them are in critical condition. He also said that authorities were still working out the number of "walking wounded." Police had previously given the total number of injured as around 40.
Rowley said investigation were continuing around Parliament but expected that lawmakers would be able to go ahead with plans to reconvene in a show of solidarity.
Before Rowley's news conference, British media reported that armed police carried out a raid on a property in Birmingham.
The Press Association on Thursday quoted an unnamed witness saying that the operation was linked to the attack. The witness said that police raided an apartment and arrested three men.
Police in the West Midlands, where Birmingham is located, directed inquiries about the operation to London's Metropolitan Police.
Lawmakers, lords, staff and visitors were locked down after the man was shot by police within the perimeter of Parliament, just yards (meters) from entrances to the building itself and in the shadow of the iconic Elizabeth Tower (commonly known as Big Ben).
The policeman who died has been identified as 48-year-old PC Keith Palmer. He reportedly died after confronting the knife-wielding terrorist. He is survived by his wife and two children.
A doctor who treated the wounded from the bridge said some had "catastrophic" injuries. Three police officers, several French teenagers on a school trip, two Romanian tourists and five South Korean visitors were among the injured.
Speaking outside 10 Downing St. after chairing a meeting of government's emergency committee, COBRA, May said that level would not change. She said attempts to defeat British values of democracy and freedom through terrorism would fail.
"Tomorrow morning, Parliament will meet as normal," she said. Londoners and visitors "will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart."
US President Donald Trump was among world leaders offering condolences, and in Paris, the lights of the Eiffel Tower were to be dimmed in solidarity with London.
London has been a target for terrorism many times over past decades. Just this weekend, hundreds of armed police took part in an exercise simulating a "marauding" terrorist attack on the River Thames.
Wednesday was the anniversary of suicide bombings in the Brussels airport and subway that killed 32 people last year, and the latest events echoed recent vehicle attacks in Berlin and Nice, France.
In the House of Commons, legislators were holding a series of votes on pensions when deputy Speaker Lindsay Hoyle announced that the sitting was being suspended and told lawmakers not to leave.
Parliament was locked down for several hours, and the adjoining Westminster subway station was shuttered.
Conservative lawmaker Tobias Ellwood, whose brother was killed in the Bali terror attack in 2002, performed first aid on the wounded police officer, who later died. About 10 yards away lay the assailant.
"I tried to stem the flow of blood and give mouth to mouth while waiting for the medics to arrive but I think he had lost too much blood," Ellwood said. "He had multiple wounds, under the arm and in the back."
The attack began early Wednesday afternoon as a driver in a gray SUV slammed into pedestrians on the bridge linking Parliament to the south bank of the River Thames.
Former Polish foreign minister Radek Sikorski was in a car crossing the bridge when he heard "something like a car hitting metal sheet" and then saw people lying on the pavement.
"I saw one person who gave no signs of life. One man was bleeding from his head. I saw five people who were at least seriously injured," Sikorski told Poland's TVN24.
Ambulances arrived within minutes to treat people who lay scattered along the length of the bridge. One bloodied woman lay surrounded by a scattering of postcards.
Police said one woman was rescued after falling into the River Thames as the attacker drove on the pavement of Westminster Bridge.
The car crashed into railings on the north side of the bridge, less than 200 yards (meters) from the entrance to Parliament. As people scattered in panic, witnesses saw a man holding a knife run toward the building.
"The whole crowd just surged around the corner by the gates just opposite the Elizabeth Tower," said witness Rick Longley. "A guy came past my right shoulder with a big knife and just started plunging it into the policeman. I have never seen anything like that. I just can't believe what I just saw."
The attacker managed to get past a gate into Parliament's fenced-in New Palace Yard, a cobbled courtyard in the shadow of the Elizabeth Tower.
Daily Mail journalist Quentin Letts said a man in black attacked the police officer before being shot two or three times as he tried to storm into the building.
"As this attacker was running towards the entrance two plain-clothed guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell," Letts told the BBC.
The attacker fell to the cobbles just yards from the entrance to 1,000-year-old Westminster Hall, the oldest part of the parliamentary complex, busy with visitors and school groups. Beyond that, a corridor leads to the building's Central Lobby, flanked by House of Commons and House of Lords chambers.
The prime minister was among lawmakers near the Commons at the time of the attack, and was quickly ushered away by security officers and driven back to Downing Street.
To get that far, the attacker would have had to evade the armed officers who patrol the Parliament complex in pairs, as well as Parliament's own security staff, who don't carry guns.
The attack unfolded near some of the city's most famous tourist sites, including the London Eye, a large Ferris wheel with pods that overlook the capital. It was halted after the attack, stranding visitors in the pods, with an aerial view of the attack scene
Israel, which that has faced a wave of Palestinian car ramming, stabbing and shooting assaults since 2015, also expressed solidarity with the victims of the London attack.
Deputy Foreign Minister Tzipi Hotovely said in a statement. "Israel expresses its deep shock at the terror attack in London today and its solidarity with the victims and with the people and government of Great Britain. Terror is terror wherever it occurs and we will fight it relentlessly."