A police officer is seen near the scene of an explosion at a waste water treatment works in Avonmouth, near the southwestern city of Bristol in western England, Britain on Dec. 4, 2020. Four people were killed Thursday in a large explosion at a waste water treatment works in Avonmouth, local police confirmed.

A police officer is seen near the scene of an explosion at a waste water treatment works in Avonmouth, near the southwestern city of Bristol in western England, Britain on Dec. 4, 2020. Four people were killed Thursday in a large explosion at a waste water treatment works in Avonmouth, local police confirmed.

Photo taken on Dec. 4, 2020 shows police vehicles near the scene of an explosion at a waste water treatment works in Avonmouth, near the southwestern city of Bristol in western England, Britain. Four people were killed Thursday in a large explosion at a waste water treatment works in Avonmouth, local police confirmed.

Photo taken on Dec. 4, 2020 shows a general view of the scene of an explosion at a waste water treatment works in Avonmouth, near the southwestern city of Bristol in western England, Britain. Four people were killed Thursday in a large explosion at a waste water treatment works in Avonmouth, local police confirmed.

A police officer moves a sign near the scene of an explosion at a waste water treatment works in Avonmouth, near the southwestern city of Bristol in western England, Britain on Dec. 4, 2020. Four people were killed Thursday in a large explosion at a waste water treatment works in Avonmouth, local police confirmed.

Photo taken on Dec. 4, 2020 shows police vehicles near the scene of an explosion at a waste water treatment works in Avonmouth, near the southwestern city of Bristol in western England, Britain. Four people were killed Thursday in a large explosion at a waste water treatment works in Avonmouth, local police confirmed.

Photo taken on Dec. 4, 2020 shows police vehicles near the scene of an explosion at a waste water treatment works in Avonmouth, near the southwestern city of Bristol in western England, Britain. Four people were killed Thursday in a large explosion at a waste water treatment works in Avonmouth, local police confirmed. (Photos: Xinhua)