LONDON, England (AP) — British authorities raised the country’s threat level to its second-highest rung yesterday after police said a blast in a taxi outside a Liverpool hospital that killed one man and injured another was caused by a homemade bomb.

Investigators said they were treating the explosion in Liverpool as a terrorist incident, but that the motive was unclear.

The Joint Terrorism Analysis Centre raised the UK threat level from substantial — meaning an attack was 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 to those who seek to divide us with senseless acts of violence,” he said.

A 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, head of Counterterrorism Policing in north-west 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 under the Terrorism Act on Sunday, and a fourth was detained on yesterday. 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 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.

He said the passenger had been picked up by the cab a 10-minute drive away and asked to be taken to the hospital, where the explosion occurred. The driver, named by local media as David Perry, managed to escape from the car. He was treated in the hospital and released.

Police said officers had searched two addresses in the city linked to the passenger and found “significant items” at one of them,

Liverpool Mayor Joanne Anderson said the taxi driver locked the doors of his cab so the passenger couldn’t leave. Police did not confirm that account.

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

The prime minister also said the driver appeared to have behaved “with incredible presence of mind and bravery”.

Nick Aldworth, a former senior terrorism investigator in Britain, said the taxi appeared to have sustained “a lot of fire damage with very little blast damage.”

He said, “whatever was in that vehicle was either a low yield or didn’t work properly, or possibly an incendiary. So I think it’s very much open to debate at the moment about what has happened.”

Britain’s official terrorism threat level was lowered from “severe” to “substantial” in February. It has been at severe most of the time since 2014, briefly rising to “critical” amid a spate of violent attacks in 2017.

The Joint Terrorism Analysis Centre sets the threat level based on intelligence about international terrorism at home and overseas.

Britain has experienced attacks by both Islamic and far-right extremists over the years, including a May 2017 suicide bombing at an Ariana Grande concert in Manchester that killed 22 people.

Amess was stabbed to death October 15 while meeting with constituents, in what police said was an act of terrorism by a supporter of the Islamic State group. The slaying came five years after Labour Party lawmaker Jo Cox was shot and stabbed to death by a far-right extremist.