Britain raised its terrorism threat level on Monday after a deadly explosion in a taxi was declared a terrorist incident.
The explosion engulfed a taxi in front of a hospital in Liverpool, northern England, on Sunday. The blast killed a male passenger in the taxi, who had asked to be driven to the hospital.
Security camera footage showed the vehicle suddenly exploding as it approached a drop-off point in front of the hospital. It also showed the taxi driver escaping from the car.
Police said the 32-year-old passenger was killed and the driver was injured.
Police suspect that an improvised explosive device carried into the vehicle by the passenger may have blown up.
Police have arrested four men in their 20s in connection with the incident.
The BBC and other local media reported that the dead man was a refugee from Syria who was not known to British intelligence authorities.
Interior Minister Priti Patel announced that the country's threat level had been raised to the second-highest on a five-stage scale in the wake of the terrorist act.