An explosive device that blew up a taxi in the northern English city of Liverpool on Sunday was carried on board by a passenger and the blast is being treated as a terrorist incident, police said on Monday.
Police said they believed they knew the identity of the passenger, who was killed by the explosion, but could not disclose it.
"Although the motivation for this incident is yet to be understood, given all the circumstances, it has been declared a terrorist incident and counter-terrorism policing are continuing with the investigation," said Assistant Chief Constable Russ Jackson of Counter-Terrorism Policing Northwest.
The taxi exploded outside Liverpool Women's Hospital, killing the passenger and injuring the driver who received medical treatment but has since been released.
"Remarkably, the taxi driver escaped from the cab," Jackson said.
Three men aged 29, 26 and 21 were arrested on Sunday, and police said on Monday another man aged 20 had been arrested.
Jackson told reporters that the taxi driver had picked up the passenger, who had asked to be driven to the hospital. He said it was unclear why he wanted to go to the hospital or what had caused the sudden explosion of the device.
The explosion took place a minute before the start of a Remembrance Day service to commemorate war dead at nearby Liverpool Cathedral.
"We are of course aware that there were Remembrance events just a short distance away from the hospital and that the ignition occurred shortly before 11 a.m.," said Jackson.
"We cannot at this time draw any connection with this but it is a line of inquiry which we are pursuing."