British authorities raised the threat level to its second-highest rung on Monday, after police said a blast in a taxi outside a Liverpool hospital was caused by a homemade bomb.
Investigators said they were treating Sunday's explosion – which killed the suspected bombmaker and injured the cab driver – as a terrorist incident, but that the motive was unclear.
Counterterrorism police named the dead man as 32-year-old Emad Al Swealmeen. They did not give further details. But Britain's Press Association news agency and other media reported that he had not been on the radar of the security services.
The Joint Terrorism Analysis Centre raised the UK threat level from substantial – meaning an attack is likely – to severe, meaning it is highly likely, following the UK's second fatal incident in a month.
Conservative lawmaker David Amess was stabbed to death in October in what police said was an act of terrorism.
Prime Minister Boris Johnson condemned the "sickening attack" at Liverpool Women's Hospital and told reporters that the British people "will never be cowed by terrorism."
'We will never give in'
"We will never give in to those who seek to divide us with senseless acts of violence," he said.
The male passenger in a taxi was killed and the driver injured when a blast ripped through the vehicle as it pulled up outside the hospital on Sunday morning.
Russ Jackson, the head of Counterterrorism Policing in northwest England, said the explosion was caused by "the ignition of an explosive device" that was brought into the vehicle by the passenger.
Three men in their 20s were arrested elsewhere in the city on Sunday under the Terrorism Act and a fourth was detained on Monday. All are believed to be "associates" of the dead passenger, police said.
Suspicions about a motive for the explosion have been aroused by the timing – just before 11 am on Remembrance Sunday, the moment when people across Britain hold services in memory of those killed in wars.
Jackson said investigators had not found a link to remembrance events "but it is a line of inquiry we are pursuing."
"Although the motivation for this incident is yet to be understood, given all the circumstances, it has been declared a terrorist incident," he said at a press briefing.