The upgrade in the threat level on Monday from substantial to severe indicates the authorities believe an attack is highly likely. One man died when a taxi blew up outside Liverpool Women's Hospital on Sunday.
British authorities have raised the country's terrorism threat level to "severe" after the explosion of a taxi outside a hospital was ruled a terrorist incident.
The upgrade in the threat level on Monday from "substantial" to "severe" indicates the authorities believe an attack is highly likely.
It comes a day after a blast at Liverpool's Women’s Hospital which Russ Jackson, the head of Counterterrorism Policing in northwest England, said involved an improvised explosive device.
The explosion on Sunday morning killed the taxi's male passenger and injured the driver. The blast was caught on video, which shows the driver fleeing the vehicle after it blew up, shortly before it burst into flames.
Police said in a statement that their enquiries indicate "that the [explosive] device was brought into the cab by the passenger".
Taxi passenger identified, four men released
They named the passenger on Monday evening as 32-year-old Emad Al Swealmeen. They did not give further details, though Britain’s Press Association news agency and other media reported that he had not been on the radar of the security services.
He is said to have hired the taxi, telling the driver to take him to the hospital, a 10-minute journey by car.
The driver, named by local media as David Perry, managed to escape from the car when it blew up. He was treated in the hospital and released.
Police said overnight on Monday that four men detained under the Terrorism Act had been released. The men — aged 29, 26, 21, and 20 — had been arrested in the Kensington area of Liverpool.
"Following interviews with the arrested men, we are satisfied with the accounts they have provided and they have been released from police custody," Greater Manchester Police said in a statement.
"The investigation continues to move at a fast pace with investigative teams working throughout the night."
The statement added that significant progress had been made since Sunday morning and police had a greater understanding of the explosive device and how it had been assembled. Important evidence had been recovered from an address which "is becoming central to the investigation".
"We are gaining a better understanding by the hour but it is likely to be some time, perhaps many weeks until we are confident on our understanding of what has taken place."
PM condemns 'sickening' attack
Prime Minister Boris Johnson described the attack as "sickening" during a televised press conference on Monday afternoon, originally called to discuss the COVID-19 pandemic.
"My thoughts remain will all those affected, and, on behalf of the whole country, I want to pay tribute to the emergency services who responded as always with such speed and professionalism," Johnson said.
"But what yesterday showed above all is 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, and our freedoms and our way of life will always prevail," he added.
Britain's Home Secretary (interior minister) Priti Patel stressed that the attack was the second in a month, following the murder of MP David Amess on October 15 as he held a constituency surgery in Leigh-on-Sea, 60 kilometres from London. A man has been charged with murder and with preparing terrorist acts.
The explosion occurred just before 11 am on Remembrance Sunday, the time people across Britain traditionally pause in a two-minute silence in memory of those killed in wars.
Police urged the public to "remain calm but vigilant."