Avon and Somerset Police Chief Inspector said the incident was not terror-related The explosion at the plant operated by Wessex Water occurred in a silo that holds treated bio-solids before they are recycled as organic soil conditioners LONDON: Four people died on Thursday when a chemical tank exploded at a waste water treatment plant in western England, police said. "We can confirm there have been four fatalities," Avon and Somerset Police Chief Inspector Mark Runacres said after the explosion at the plant at Avonmouth, near Bristol. A fifth person was injured but their injuries were not said to be life-threatening, he added. Police said they would not be speculating on the cause of the explosion but added it was not being treated as terror related. Prime Minister Boris Johnson tweeted that he was "deeply saddened to learn that four people have lost their lives". "Our hearts go out to the victims and their families. Thank you to the emergency services who attended the scene," he added. The explosion at the plant operated by Wessex Water occurred in a silo that holds treated bio-solids before they are recycled as organic soil conditioners, added Runacres. Witness Jawad Burhan said there was a "helicopter looking for missing people" and police closed a nearby road leading up to the plant. "I heard the sound, I"m working beside the building in another warehouse. After 10 minutes I saw the helicopter coming and the police," he said. Another witness, Kieran Jenkins, told the BBC he was in a nearby warehouse when the explosion occurred, saying it was "shaking and we literally stood there in shock. "Next thing you know we looked out of the windows and all we could see was people running. "We don"t know what happened. It was a bit of a shock really. I heard a bang... we didn"t know what was going on." An investigation has been launched into the blast with the Health and Safety Executive.