LONDON — The British government raised its terrorism threat level to its second-highest level on Monday, after police labeled two attacks in the past month — one car bomb outside a Liverpool hospital, and one stabbing of a British lawmaker — terrorist incidents.

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       The government announced it was upgrading its terrorist threat level from “substantial” to “severe” — meaning that the government considers the possibility of an attack “highly likely.”

       The move came after British police arrested four men under the country’s terrorism legislation following Sunday’s car explosion, which killed one person and injured another outside a hospital in northwestern England. The Greater Manchester Police said the men, ages 29, 26, 21 and 20 were arrested in Kensington, an area of Liverpool that has a high poverty rate.

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       Authorities did not provide any further information about the evidence against them.

       Prime Minister Boris Johnson’s office said he will chair a meeting of COBRA, the government’s crisis committee, on Monday afternoon in response to the attack.

       British police declare taxi explosion outside Liverpool hospital ‘terrorist incident’

       Last month, a 25-year-old man was charged under terrorism legislation after the fatal stabbing of Conservative lawmaker David Amess.

       Amess, 69, who represented Southend West in Essex, was meeting with constituents in a church building in his home district when he was stabbed on Oct. 15.

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       Authorities are still investigating Sunday’s car explosion. Russ Jackson, head of counterterrorism policing in northwestern England, said Monday that “although the motivation for this incident is yet to be understood, given all the circumstances, it has been declared a terrorist incident and counterterrorism policing are continuing with the investigation.”

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       The blast occurred on Sunday just before 11 a.m. local time as a taxi pulled up outside an entrance of Liverpool Women’s Hospital. Video footage shared on social media shows the blast ripping through the vehicle, which then bursts into flames. One passenger, who has not been publicly identified, was declared dead at the scene.

       The taxi driver picked up a passenger about 10 minutes away from the hospital, and an explosion took place occurred within the car as the taxi approached the drop-off point, police said. Jackson said the incident involved the ignition of an explosive device that was brought into the cab by the passenger.

       “Our enquiries will now continue to seek to understand how the device was built, the motivation for the incident and to understand if anyone else was involved in it,” Jackson said in a statement he read out at a news briefing.

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       The driver, who was able to escape, was hospitalized in stable condition and was subsequently released.

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       The driver was praised for his apparent efforts to limit the impact of the explosion in his vehicle by reportedly locking the passenger inside the cab.

       “It does look as though the taxi driver in question did behave with incredible presence of mind and bravery,” Johnson said Monday, noting the investigation is ongoing and said it was premature to share further details.

       Questions have been raised about the timing of the explosion, which occurred on Remembrance Sunday, a day when Britain pays tribute to its war dead by observing two minutes of silence starting at 11 a.m. Officers said they have not drawn any link between the occasion and the incident but that it remains a “line of inquiry.”

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       — Miriam Berger contributed to this report from Washington and Rachel Pannett from Sydney.

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