U.K. police have declared Sunday's Liverpool car blast a terrorist act

Police are investigating a deadly car explosion in the English city of Liverpool as a terrorist incident, British authorities said on Monday.

They have also arrested a fourth person in connection with Sunday's blast, in which one person died and another was injured after an explosive device went off in a taxi outside Liverpool Women's Hospital. Police later said Emad Al Swealmeen, 32, died in the explosion.

"Although, the motivation for this incident is yet to be understood, given all the circumstances, it has been declared a terrorist incident and Counter Terrorism Policing are continuing with the investigation," Assistant Chief Constable Russ Jackson, the head of Counterterrorism Policing in northwest England, said in a statement on Monday.

According to police, a local taxi driver picked up a male passenger in the Rutland area of Liverpool shortly before 11 a.m. local time on Sunday. The passenger asked to be taken to Liverpool Women's Hospital, about 10 minutes away.

Liverpool Women's Hospital specializes in the health of women and babies, and the National Health Service describes it as the largest women's hospital of its kind in Europe.

"As the taxi approached the drop-off point at the hospital, an explosion occurred from within car, which quickly engulfed it in flames," authorities said.

Emergency responders put out the fire and discovered that the passenger was inside the vehicle and was dead. The taxi driver managed to escape and has since been released from the hospital after being treated for injuries.

The taxi driver is being hailed as a hero

The taxi driver has garnered praise from local and national leaders for his conduct.

Liverpool Mayor Joanne Anderson told BBC Radio 4's Today that he had gotten out of the taxi and locked the doors before the blast, though the BBC reports that Jackson has not confirmed that account.

"The taxi driver, in his heroic efforts, has managed to divert what could have been an absolutely awful disaster at the hospital," Anderson said.

British Prime Minister Boris Johnson also told reporters on Monday that the driver appeared to have acted with "incredible presence of mind and bravery."

The motive is still unclear

Investigators' working assumption is that the improvised explosive device was brought into the taxi and manufactured by the passenger, Jackson said, but they don't yet know why he took it to the Women's Hospital or the reason behind its sudden explosion.

The motive behind the blast is not year clear. Police noted that Remembrance Day events were taking place at a cathedral near the hospital and said that is one line of inquiry they are pursuing.

"We believe we know the identity of the passenger but cannot confirm this at this time," they added.

Police have arrested four men and searched two addresses

Jackson said the investigation has led police to two addresses, both in the Kensington area of Liverpool.

Police arrested three men ages 21, 26 and 29 at one of those locations on Sunday on terrorism-related charges. A 20-year-old man was later arrested in the same area. They will be interviewed by authorities on Monday.

Authorities are also searching both addresses, Jackson added. At the second location, police found "significant items" that Jackson said will necessitate further searches on Monday and perhaps the coming days. Eight families have been evacuated from that address.

"Our inquiries 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, urging members of the public to come forward with any information they may have.

The hospital is open with safety restrictions

Meanwhile, hospital officials on Monday said patients are no longer being diverted to other hospitals, and they provided a list of safety precautions they are taking as the investigation continues.

For example, they are restricting visiting access until further notice, increasing police presence on site, limiting access to the building through one entry point and asking visitors to park their cars in designated areas.

This story originally appeared on the Morning Edition live blog.

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