British authorities have declared the car explosion outside a hospital in Liverpool a terrorist incident. The explosion of a taxi outside the medical facility killed one man and injured another. Police said Monday that the explosion was caused by an improvised explosive device.

The head of Counterterrorism Policing in northwest England, Russ Jackson, said the IED was brought inside the taxi by a yet to be identified passenger. The device then exploded when the taxi stopped in front of the Liverpool Women’s Hospital.

The passenger of the taxi cab died on the scene, while the driver was able to escape before the fire engulfed the entire vehicle. The driver is being treated at a nearby hospital, and he is reportedly in stable condition.

The UK has set the threat level for the latest attack in the middle of its five-point scale, which means that a subsequent attack is “likely.”  The threat level is based on intelligence regarding international terrorism at home and abroad as determined by the Joint Terrorism Analysis Center.

Police said the taxi picked up the passenger 10 minutes before the explosion. The passenger reportedly asked to be taken to the hospital. Investigators said the driver, a man named David Perry, locked the doors of his car to prevent the passenger from escaping.

Liverpool Mayor Joanne Anderson called the driver a “hero” for his actions. Anderson said Perry managed to avert a much larger disaster from happening at the hospital.

On Sunday, three men in their twenties were arrested by police under the Terrorism Act, while a fourth was apprehended on Monday.

The timing of the explosion – shortly before 11 a.m. — has sparked suspicions about its motivation. The explosion occurred on Remembrance Sunday when people around the UK honored those who had died in previous wars.