Four people have been arrested over an explosion outside Liverpool Women's Hospital.
What do we know about the circumstances leading up to the explosion?
Police said that shortly before 11am on Remembrance Sunday, a taxi picked up a fare in the Rutland Avenue area of Liverpool.
The passenger asked to be taken to Liverpool Women's Hospital, a journey of about 10 minutes.
When the vehicle approached the drop-off point outside the hospital, there was an explosion inside the car.
As smoke and flames engulfed the vehicle, the taxi driver - identified as David Perry - managed to escape. He was injured, but later released from hospital after treatment.
The male passenger inside the taxi was killed by the explosion.
Police make arrests as investigation begins
At about 1pm on Sunday - approximately two hours after the explosion - armed police raided a home in Rutland Avenue near Sefton Park.
Residents were evacuated as police set up a large cordon.
Then at about 2pm, another property was raided by armed police in the Kensington area of Liverpool.
Three men, aged 29, 26 and 21, were arrested in a house in Sutcliffe Street, which is about a mile from the hospital.
Sutcliffe Street and the adjacent Boaler Street were also cordoned off by police.
On Monday, a fourth man, aged 20, was also arrested in the Kensington area.
Two addresses in total, one in Sutcliffe Street and another in Rutland Avenue, have been searched, with the second address yielding "significant items", police said.
Later on Monday morning, police declare the explosion as a terror incident.
What caused the explosion - and what was the motivation?
Officers believe the passenger took an improvised explosive device into the taxi, but they are unsure what the motivation behind the attack was or the reason for the device's "sudden explosion".
Assistant Chief Constable Russ Jackson, from Counter-Terrorism Policing North-West, said: "Our enquiries indicate that an improvised explosive device has been manufactured and our assumption so far is that this was built by the passenger in the taxi."
Police said they are "aware there were Remembrance events just a short distance from the hospital" and it is a "line of inquiry we are pursuing".
What impact is there on the investigation following the police declaration that it is a terrorist incident?
It means the investigative officers will be able to call on the support of the intelligence services and it will allow them to "start exploring" any international connections.
Former Counter Terrorism National Coordinator, Nick Aldworth, told Sky News: "It's about what resources you can bring into the investigation.
"By declaring it as a terrorist incident early on will have meant that they could have brought in the intelligence services to support the investigation.
"The intelligence services hold their own information, some of which is shared with police and some of which is not."
MI5 is assisting the police with the investigation.
What is known about the taxi passenger killed in the blast?
Mr Jackson said officers believe they "know the identity of the passenger" but could not confirm any further details "at this time".
He added the arrested men were believed to be "associates" of the taxi passenger.
Mr Jackson also said police had "attributed" the taxi passenger to both the addresses where officers were currently searching, but were uncertain which address he lived at.
The passenger is not currently thought to have been on the radar of the security services, Sky News understands.
What has been the reaction in Liverpool?
Sharon Cullen, 49, was at home in Boaler Street on Sunday afternoon when she looked out of her rear window and saw armed officers surrounding a property in Sutcliffe Street.
Mrs Cullen said: "I saw around five or six of them and two went round the back and put ladders against the wall and climbed up."
Police had earlier told her family to stay indoors. She added: "It was terrible. It's not how I wanted to spend my Sunday afternoon."
Liverpool mayor Joanne Anderson said the community is "very good at pulling together".
She also lauded the driver, Mr Perry, for his "heroic efforts".