The UK’s terror threat level has been raised from “substantial” to “severe”, meaning an attack is now judged to be “highly likely”.
The move, confirmed by Home Secretary Priti Patel, follows an explosion outside a hospital in Liverpool on Sunday which police have declared a terror incident.
Detectives have arrested four people over the blast, which killed a passenger inside a taxi.
Police believe it was that passenger who took an improvised explosive device into the vehicle, which exploded as the taxi arrived outside the reception of Liverpool Women’s Hospital.
The decision to raise the threat level has been taken by the Joint Terrorism Analysis Centre (JTAC) and Ms Patel said it was made due to two incidents occurring in the last month.
The explosion outside Liverpool Women’s Hospital on Sunday has seen the UK’s terror threat level being lifted to severe – which indicates that an attack somewhere in the UK is “highly likely”.
One man was killed and a taxi was destroyed in the blast – with the driver needing hospital treatment. Four men have been arrested under the Terrorism Act.
Amid a fast-moving investigation, here’s what we know so far.
Merseyside Police were called at 10:59 GMT to reports of a car explosion at Liverpool Women’s Hospital.
A taxi carrying one passenger – a man who had been picked up from the Rutland Avenue area and asked to be taken to the hospital about 10 minutes away – pulled up to the hospital and exploded shortly afterwards.
The passenger, who was declared dead at the scene, is yet to be formally identified.
However, Russ Jackson, head of Counter Terrorism Policing North West, said officers believed they knew his identity but could not confirm this at the moment.
The passenger appeared to have made an improvised device that caused the explosion, he said.
He said officers investigating the incident could not “at this time draw any connection” with Remembrance Sunday events taking place near the hospital but he confirmed this was “a line of inquiry”.
The taxi driver – named locally as David Perry – was treated for injuries but has now been released from hospital.
Joanne Anderson, the mayor of Liverpool, told BBC Radio 4’s Today programme the driver involved prevented an “awful disaster” through heroic actions.
She said it was known he had got out of the car and locked the doors before the explosion happened – but she did not want to get drawn into further speculation.
Mr Jackson said he could not confirm reports the driver had locked the doors of the taxi before the explosion. While he had spoken to the driver, at the time he was still shaken and injured and police have not yet got a full account of the incident, he added.
He said it appeared to have been an “unremarkable journey” to the hospital.
Police, firefighters and a bomb disposal unit attended the scene.
Phil Garrigan, chief fire officer of Merseyside Fire and Rescue Service, said the fire was “fully developed” when two appliances arrived shortly after 11:00.
Nick Aldworth, a former counter terror co-ordinator, said investigators would be focused on what happened inside the vehicle.
“They’ll be looking at what sort of damage has been caused, trying to get an assessment of what might have caused that blast,” he told Today.
“I have to say from what I’ve seen there is very little blast damage – a lot of fire damage but very little blast damage. So whatever was in that vehicle was either a low yield or didn’t work properly or was possibly an incendiary.”
About two hours after the blast, properties on Rutland Avenue near Sefton Park and in the Kensington area were raided by armed officers.
Three arrests took place in Sutcliffe Street, in Kensington, where witnesses reported seeing armed officers approach a terraced house.
Jim Clarke, BBC North West Tonight deputy editor, said the first raid happened at about 13:00 on Rutland Avenue – roughly 0.75 miles from the hospital.
The second around Sutcliffe Street and the adjacent Boaler Street – about a mile from the hospital – took place an hour later, at about 14:00.
There was a heavy police presence, including police negotiators, outside the house on Rutland Avenue overnight.
Several residents were evacuated and a large cordon guarded by officers was put in place.
Police have “attributed” the taxi passenger to the two addresses, which officers are currently searching, and they have found “significant items” at one of these, Mr Jackson said.
Three men – aged 29, 26 and 21 – were arrested under the Terrorism Act after being detained in the Kensington area of the city on Sunday.
Officers from Counter Terrorism North West are leading the investigation, supported by Merseyside Police and security service, MI5.
On Monday, a fourth man, aged 20, was arrested in Kensington under the Terrorism Act.
People detained under the Terrorism Act can be held without charge for up to 14 days.
Mr Jackson said the arrested men were believed to be “associates” of the taxi passenger.
He said officers were looking at his associates, telephone records and purchases he may have made.
The Chief Constable of Merseyside Police, Serena Kennedy, offered her reassurance.
“There is no specific threat to the area, but I have asked for patrols to be increased right across Merseyside,” she said.
The change in the UK terror threat level to severe, the second highest, means a further attack is considered “highly likely”.
Home Secretary Priti Patel said the change was because the Liverpool blast was the “second incident in a month” following the killing of MP David Amess in Essex.
Liverpool Women’s Hospital said anyone with appointments at the hospital would notice an increased security and police presence on site. They asked anyone coming to the hospital to avoid bringing their vehicle for the time being.
The hospital is a major obstetrics, gynaecology and neonatology research centre – the largest for its specialism in Europe. It featured on the popular Channel 4 programme One Born Every Minute.
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