Nov. 15 (UPI) -- Authorities in Britain said Monday that a taxi that exploded in Liverpool, which killed at least one person, was a terrorist attack.
The explosion occurred on Sunday outside of a Liverpool hospital, police said, after a passenger took an improvised explosive device into the taxi.
Investigators said that four men have been arrested and that "significant items" were found at two addresses searched by police. The man believed to have carried the explosive was arrested Monday.
Taxi driver David Perry escaped the blast and was briefly hospitalized. Authorities said one person was killed.
The motive for the incident remains unclear.
"Our police and emergency services are working hard to establish what happened and it is right they are given the time and space to do so," Priti Patel, Britain's home secretary, tweeted.
A large area near the hospital was cordoned off after the attack and several addresses were evacuated as a precaution.
Chief Constable Serena Kennedy said that such explosions are "very rare" and urged the public to avoid speculation.
"There will also be an increased and visible police presence on the streets of Merseyside and I would encourage people to engage with my officers and raise any concerns they may have," Kennedy said in a tweet. "We will endeavor to update our communities as soon as we are able."
British Prime Minister Boris Johnson tweeted that his thoughts are with those affected by the "awful incident and Liverpool Mayor Joanne Anderson called the blast "unsettling."