Four people have been killed and one person injured after an explosion at a wastewater treatment plant in Bristol that could be heard more than a mile away. It is believed the five people were all working on top of or near a silo that contained biosolids – solid matter recovered in sewage treatment processes and turned into organic soil conditioner. A rescue operation involving urban search and rescue teams, tracker dogs and helicopters was launched following the explosion at the Wessex Water site in Avonmouth. Avon and Somerset police confirmed on Thursday evening that everyone had been accounted for and reassured people living nearby that there was not believed to be any further danger to the public. They said they did not believe the incident was terror-related and agencies including the Health and Safety Executive (HSE) would be involved in the investigation. Police refused to speculate on the cause of the explosion but Avon fire and rescue service said it believed the silo also contained lime, which can be used in the stabilisation process of waste treatment. The Wessex Water chief executive, Colin Skellett, said: “We are absolutely devastated that the tragic incident at our site has resulted in four fatalities. Our thoughts are with the families, friends and colleagues of those involved. We are working with the Health and Safety Executive to understand what happened and why.” The prime minister and home secretary both expressed their concern. Boris Johnson said: “Deeply saddened to learn that four people have lost their lives in the water works explosion in Avonmouth. Our hearts go out to the victims and their families. Thank you to the emergency services who attended the scene.” Priti Patel said she was “deeply concerned”. Bristol’s mayor, Marvin Rees, said: “The thoughts of our city are with their families at this heartbreaking time. This has already been such a challenging year, and this news of further loss of life is another terrible blow. As a city we will mourn for them. Local residents may be concerned about safety in the area. Wessex Water, police and fire have assured us the incident is contained, and there is no risk to your safety.” Ch Insp Mark Runacres, of Avon and Somerset police, told a press conference at the scene that three employees of Wessex Water and one contractor had died. The fifth person’s injuries were not life-threatening. He said: “Emergency services were called at approximately 11.20am to reports of a large explosion involving one of the chemical tanks at the site off Kings Weston Lane. The fire service led the rescue operation but sadly, despite the best efforts of all those involved, we can confirm there have been four fatalities. “Specially trained officers have this afternoon made contact with each of the families of those individuals and informed them of the sad news. A thorough investigation will be carried out into what happened today, involving a number of agencies and the Health and Safety Executive in due course, and we will not be speculating on the cause of the explosion while such work is being undertaken. “I can confirm though that this is not being treated as a terror-related incident. The explosion happened in a silo that holds treated biosolids before it is recycled to land as an organic soil conditioner. I can reassure people in the nearby area there is not believed to be any ongoing public safety concerns.” Luke Gazzard, the incident commander for Avon fire and rescue service, said firefighters were on the scene within five minutes of the explosion. He described it as a “very challenging incident”. Gazzard said there had been no fire, only the powerful explosion. Darren Jones, the Labour MP for Bristol North West, said lessons needed to be learned. He said it came just weeks after a fire involving scrap metal burned for three days at Avonmouth. “We have to ensure these regulated environments are kept up to spec,” he said. Sean Nolan, who was working nearby, said he heard a “boom and echo” and at first thought two trucks had collided. “Then it just went quiet. That was it. There was no smoke, there was no after-effects of it.”