In Pictures: Terror Attack Outside British Parliament In London

London police say at least four people were killed in a March 22 terrorist attack at the British Parliament -- including a suspected attacker, a police officer stabbed by an assailant, and two people struck by the assailant's vehicle when he rammed into pedestrians on Westminster Bridge.

Police said they believe a lone assailant carried out attack, which left at least 20 others injured, but that they were continuing a counterterrorism investigation and had not ruled out the possibility that others may have been involved.