UK police have declared the blast outside a hospital in Liverpool a terrorist incident and said they believed the explosion occurred when a man brought an improvised device into a taxi.

Officers said Monday that a passenger is believed to have entered the vehicle with an explosive device that suddenly went off. The suspect died in the blast but the taxi driver managed to escape. Police have arrested four men, all aged in their 20s, and believe they know the identity of the suspect.

The incident began when the driver of the taxi picked up a fare on a residential street in the northwest English city. A man entered and asked to be taken to Liverpool Women’s Hospital, a short drive away, Assistant Chief Constable Russ Jackson of Counter Terrorism North West said at a press conference on Monday.