British police have been working through the night to investigate the deadliest terror attack carried out in central London in 12 years, in which four people died and scores were injured.
Investigators were "working to look at associates" of the suspect who rammed a car into a crowd on Westminster Bridge before attempting to storm the Houses of Parliament, Mark Rowley, Britain's most senior counterterror police officer, told journalists late Wednesday.
Rowley said police believe it was an act of "Islamist-related terrorism," and indicated they knew the identity of the assailant but were not releasing his name.
Overnight, police in the city of Birmingham, in central England, raided an apartment, but it was not clear whether it was linked to the London attack. When asked for information, local police referred CNN to London's Metropolitan Police. The Met told CNN it would not comment for "operational reasons."
Only one victim has been publicly identified. Keith Palmer, a 15-year veteran of the London police force, was fatally stabbed on the grounds of Parliament before police killed his attacker.
A number of tourists were among 40 people hurt in the assault, including five South Koreans and three French high school students, according to officials from both countries. One Australian had been hospitalized, officials there said.
It was the first mass-casualty terrorist attack in Britain since 2005 when 52 people and four attackers died in the July 7 bomb attacks on the London public transportation system.
The area around Westminster was teeming with people when the attack began at about 2:40 p.m. local time (10.40 p.m. ET). Witnesses said the attacker drove his car over Westminster Bridge and into the crowd. The vehicle hit a large number of people, including three police officers.
"The car then crashed near to Parliament and at least one man -- armed with a knife -- continued the attack, trying to enter Parliament," Rowley said. One of those who died was a woman, he added.
Parliament was placed on lockdown for several hours and lawmakers were forced to remain in the main debating chamber of the House of Commons as officers rushed to the scene outside.
In the early hours of Thursday, much of Westminster -- Britain's political heart -- was still cordoned off by police, with access blocked to Downing Street and police headquarters at Scotland Yard.(Source: CNN)