An explosion outside Liverpool Women’s Hospital on Remembrance Sunday has been declared a terror incident by police.
A taxi exploded and was engulfed in flames just before 11:00 GMT at a drop-off zone near the entrance, killing the passenger and injuring the driver.
The cabbie, named locally as David Perry, was declared a hero by Liverpool Mayor Joanne Anderson for locking the suspect inside the vehicle.
Four men have been arrested in the city under the Terrorism Act.
Head of Counter Terrorism Policing North West, Russ Jackson, said the taxi passenger appeared to have made an “improvised explosive device” which caused the blast.
The man’s motivation was “yet to be understood”, he added.
Assistant Chief Constable Jackson said police knew the identity of the attacker but would not confirm it at this stage.
Prime Minister Boris Johnson will chair an emergency Cobra meeting on Monday afternoon in response to the explosion, Downing Street said.
Mr Jackson said he understood Delta Taxis driver Mr Perry picked up a passenger in Rutland Avenue, near Sefton Park, on Sunday.
“The fare – a man – had asked to be taken to Liverpool Women’s Hospital, which was about 10 minutes away,” he added.
“As the taxi approached the drop-off point at the hospital an explosion occurred from within the car. This quickly engulfed it in flames.
“Remarkably the taxi driver escaped from the cab.
Mr Jackson said the taxi driver had been treated for his injuries and had been released from hospital.
He said police were “aware there were Remembrance events just a short distance away from the hospital and that the ignition occurred shortly before 11am”.
“We cannot at this time draw any connection with this but it is a line of inquiry we are pursuing,” he added.
Police confirmed a fourth man, aged 20, was arrested earlier in the Kensington area of the city and was in custody.
Three other men – aged 21, 26 and 29 – were detained on Sunday in Sutcliffe Street in Kensington.
Mr Jackson said the arrested men were believed to be “associates” of the taxi passenger.
He said officers were looking at his associates, his telephone records and purchases he may have made.
All four men in custody will be interviewed later by counter-terrorism detectives.
A cordon remains in place outside a terraced house in Sutcliffe Street.
A grey Mondeo car parked adjacent has been taped off as uniformed officers continue to guard the front and side alley at the property.
Mr Jackson said police had “attributed” the taxi passenger to both the addresses where officers were currently searching but were uncertain which address, he lived at.
He said “significant items” were found at the second property in Rutland Avenue.
The address was searched overnight and further searches are due to take place later by officers.
A cordon is also in place at this location and eight families have been evacuated from their homes.