LONDON — A knife-wielding man went on a deadly rampage in the heart of Britain’s seat of power Wednesday, plowing a car into pedestrians on London’s Westminster Bridge before stabbing a police officer to death inside the gates of Parliament. Five people were killed, including the assailant, and 40 others were injured in what 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. He died, as did three pedestrians on the bridge, and the police officer. 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. Forty others, including three police officers, were injured.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.