Seven suspects were taken into custody in Britain as police on Thursday put the death toll in the terror attack in the heart of London at four.
Seven of the people wounded in the Wednesday attack near Parliament were still in hospital and in critical condition, police said. Twenty-nine were discharged from hospitals.
Mark Rowley, Acting Deputy Commissioner and head of Counter-Terrorism unit, said the dead included a woman in her 40s, a man in his 50s, police officer Keith Palmer and the attacker.
Most media reports earlier said three members of the public were killed when the lone attacker, who has not been identified and who was later shot dead, ran over pedestrians over the iconic Westminster Bridge.
He then stabbed the unarmed Palmer on the grounds of Parliament.
Rowley said hundreds of detectives worked through the night and carried out searches at six addresses, the BBC reported. Inquiries were held in London, Birmingham and other parts of the country.
At least 40 people were hit on the crowded bridge when the attacker drove a 4×4 along the pavement, knocking down pedestrians before crashing into a fence below Big Ben.
One woman fell into the Thames from the bridge but was said to be alive, media reports said.
The intruder, described by a witness as “middle-aged and Asian”, broke into the grounds of Parliament where he fatally stabbed the police officer with two knives.
The policeman died at the scene. The attacker, shot by other police officers, died after he was taken to hospital.