Four people died and a fifth was injured after a large explosion at a sewage recycling plant near the city of Bristol in south-west England.
The group of people were standing on top of a chemical tank when it exploded.
"Sadly, despite the best efforts of all those involved, we can confirm that there have been four fatalities," Chief Inspector Mark Runacres of Avon and Somerset Police told reporters outside the site in Avonmouth in southwest England.
Three of the dead were workers at Wessex Water, which operates the plant, while the fourth was a contractor. The fifth person injured is not in a life-threatening condition.
Police said emergency services had contacted the victims' families, and that there was no connection to terrorism or any ongoing threat to people living nearby.
The blast occurred in a silo containing what Runacres called "treated biosolids" intended for use as fertiliser. Government health and safety officials would conduct an investigation into the exact cause of the explosion, he added.
Prime Minister Boris Johnson said in a statement on social media that he was "deeply saddened" to learn of the deaths and thanked emergency services for their response.
Luke Gazzard, a senior manager at Avon Fire and Rescue Service, described the scene as "a very challenging incident".
Since 2002 Wessex Water has been owned by the Malaysian company YTL Power International.
Ambulance crews, police and firefighters were called after the blast just before 11.30am. The blast caused buildings to shake, said witnesses.
"We can confirm there are multiple casualties on site," a fire service spokesman said earlier.
Chief Inspector Mark Runacres said: “Officers remain at the scene and are likely to be for some time as we work together with other agencies in dealing with this major incident.
“Police inquiries into exactly what happened are at a very early stage and are continuing."
Witness Jawad Burhan said there was a “helicopter looking for missing people” and that police had closed a road leading up to a warehouse he believed was being used as a waste centre.
“I heard the sound, I’m working beside the building in another warehouse," he said. “After 10 minutes, I saw the helicopter coming and the police.”