Three men aged 21, 26 and 29, were detained in the Kensington area of Liverpool hours after the blast on Sunday, the police said, and a fourth man, age 20, was arrested on Monday. But they were all released later in the evening after being questioned, the police said.
While attacks of this kind are rare in Britain, the country has been hit by high-profile suicide attacks in recent years. In 2005, there was a series of suicide attacks across London that killed 52 people, and in 2017, an attack on the Manchester Arena, carried out by an Islamist extremist who had met with members of ISIS, killed 22.
According to the BBC, MI5, a British intelligence service, is involved in the Liverpool investigation. While the authorities cautioned that there was no specific ongoing threat, Britain’s national terrorism threat level was raised from “substantial” to “severe,” Prime Minister Boris Johnson said during an afternoon news briefing, indicating that another attack was “highly likely.”
Mr. Jackson, the police official, described how the blast had unfolded shortly before 11 a.m., when the taxi driver, identified by local news media as David Perry, picked up a man who asked to be taken to the Liverpool Women’s Hospital. As the taxi approached the drop-off point at the hospital, an explosion went off inside the vehicle and engulfed it in flames. Remarkably, the driver escaped with minor injuries.
An initial investigation determined that the passenger had taken an explosive device into the cab.
Security camera footage taken at the scene shows the moment the taxi pulled up at the hospital entrance, before a blast appears to blow the windows out of the vehicle. White smoke can be seen rising from the car as the driver leaps out of the front seat.