An armed British police officer stands on duty as members of the emergency services gather at the Carriage Gates entrance of the Houses of Parliament in Westminster, central London on March 22, 2017, during an emergency incident.
Three people were killed and 20 injured in a "terrorist" attack outside the British parliament Wednesday when a man mowed down pedestrians, then stabbed a police officer before being shot dead. The car struck pedestrians on Westminster Bridge, a popular spot with tourists because of its views of Big Ben, before crashing into the railings outside the heavily guarded parliament building in the heart of the British capital. / AFP PHOTO / Daniel LEAL-OLIVAS        (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)