LONDON (AP) — British armed police have carried out a raid on a property in the central city of Birmingham, after an attacker killed four people before being fatally shot by police within Parliament's grounds.

Police refused to say if the raid was linked to the rampage in the heart of Britain's seat of power. But British media including the Press Association on Thursday quoted an unnamed witness saying that the operation was linked to the attack that also injured around 40 people.

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.

A knife-wielding man drove an SUV into pedestrians on Westminster Bridge before crashing the vehicle into the gates of Parliament on Wednesday. He scaled the fences and later fatally stabbed a policeman before being gunned down by officers. He hasn't been identified. Three pedestrians were among the dead.

Prime Minister Theresa May condemned as a "sick and depraved terrorist attack."

Lawmakers, lords, staff and visitors were locked down after the man was shot by police within the perimeter of Parliament, just yards from entrances to the building itself and in the shadow of the iconic Big Ben clock tower.

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.

Police said they were treating the attack as terrorism. There was no immediate claim of responsibility.

Metropolitan Police counterterrorism chief Mark Rowley said police believed there was only one attacker, "but it would be foolish to be overconfident early on." He said an unarmed policeman, three civilians and the attacker died.

Islamic extremism was suspected in the attack, Rowley said, adding that authorities believe they know the assailant's identity but would not reveal it while the investigation was ongoing.

The threat level for international terrorism in the U.K. was already listed at severe, meaning an attack was "highly likely."

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.

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," May said.

President Donald Trump was among world leaders offering condolences.

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.