LONDON >> British police say an explosion in a taxi outside a hospital that killed a man is being treated as a terrorist incident, but the motive remains unclear.

Russ Jackson, the head of Counterterrorism Policing in northwest England, said the blast on Sunday at Liverpool Women’s Hospital involved an improvised explosive device.

He said “enquiries will now continue to seek to understand how the device was built, the motivation for the incident and to understand if anyone else was involved in it.”

The male passenger in the taxi died in the explosion, and the taxi driver was injured.

Three men in their 20s have been arrested under the Terrorism Act.

Suspicions have been aroused by the timing of the explosion — just before 11 a.m. on Remembrance Sunday, the moment people across Britain hold services in memory of those killed in wars.

Liverpool Mayor Joanne Anderson said the taxi driver locked the doors of his cab so the passenger couldn’t leave.

“The taxi driver, in his heroic efforts, has managed to divert what could have been an absolutely awful disaster at the hospital,” she told the BBC.

The cabbie, whose name has not been released, is being treated in hospital for non-life-threatening injuries.

Britain’s interior minister, Home Secretary Priti Patel, said she was “being kept regularly updated on the awful incident.”

Britain’s official threat level from terrorism stands at “substantial,” the middle rung on a five-point scale, meaning an attack is likely. The Joint Terrorism Analysis Centre sets the threat level based on intelligence about international terrorism at home and overseas.