Four people are dead, including a police officer and an attacker, and 20 others were injured after a car plowed into a crowd of people on Westminster Bridge in London on Wednesday afternoon in what officials are investigating as an act of terrorism.

The knife-wielding suspect was shot outside the Houses of Parliament after crashing the car into a gate and stabbing a police officer guarding the building. Both the attacker and officer died.

Some of the wounded are said to have suffered “catastrophic” injuries, the U.K. Press Association reported.

Parliament was placed on lockdown, according to a parliamentary official.

British police are treating the incident as terrorism.

Officials said they believe there was just one attacker but a full counterterrorism investigation is under way. (Yahoo News)