Four people have died following an explosion at a waste water treatment plant near the southwest England city of Bristol, police said Thursday.

Chief Inspector Mark Runacres of Avon and Somerset Police said that a fifth person was injured during the explosion at Avonmouth but that his condition is not thought to be life-threatening.

He said the incident was not terror-related but would not speculate on the cause of the blast.

Prime Minister Boris Johnson said “our hearts go out” to the victims of the Avonmouth tragedy and their families.

“Deeply saddened to learn that four people have lost their lives in the water works explosion in Avonmouth,” he said on Twitter.