Five men have been killed and a sixth has been seriously injured in an explosion at an industrial estate in Lincolnshire.
Police, fire and ambulance crews were called to the Broadfield Lane Industrial Estate in Boston at about 1900 BST on Wednesday.
A Lincolnshire Police spokeswoman said she believed the men had died instantly in the force of the explosion.
Claims the men may have been distilling illegal alcohol are being investigated.
A spokesman for the police force appealed for anyone with information about the men and what activity was taking place at the unit to contact officers.
The sixth man was initially treated at Boston's Pilgrim Hospital before being taken to the Queen's Medical Centre (QMC) in Nottingham.
He has since been transferred to a specialist burns unit at Queen Elizabeth Hospital, in Birmingham, where his condition was described as "very poorly".
Lincolnshire Police said it was not yet clear what had caused the blast.
Supt Keith Owen, from Lincolnshire Police, said: "Our main focus is to identify the unfortunate people who died last night.
"I don't want to speculate on whether this was or was not a distillery at this stage.
"What we're looking for at the moment are witnesses and being able to identify the persons who lost their lives last night. That's the main focus of our investigation at the moment."
BBC Lincolnshire reporter Scott Dalton, who was at the scene, said residents in the area had seen thick, black smoke coming from the industrial estate.
"Locals have been telling me they didn't even hear an explosion or even a bang," he said.
Paul Featherstone, who lives nearby, said: "I didn't hear an explosion, I just saw the smoke.
"If the explosion didn't kill them, the smoke would have done because it was that bad. It was thick, black smoke coming through the roller doors."
He added he had been told by another eyewitness that one man had run out "with his arm all ablaze".
It is understood this is the man who is now being treated at Queen Elizabeth Hospital.
Steve Moore, area manager with Lincolnshire Fire and Rescue Service, said the building appeared to have been the scene "of a very hot fire with a very rapid spread".
"When the first crews attended they were faced with a really serious fire and a casualty who was outside the building suffering from burns.
"Their initial reaction was to treat the casualty. The incoming crew then started to fight the fire, it was a really hot, intense fire."
Crews were forced to use hydraulic equipment to get into the unit.
"The intensity of the fire buckled the roller shutters and set alight a car outside," he said.
He added: "For all the firefighters I've spoken to this is the most significant loss of life in a fire that they've ever attended."
Mr Moore said it was still unclear what had caused the fire, but the intense heat and rapid spread suggested some form of accelerant.
"At this stage we cannot say what caused such intense heat or fire spread.
"Certainly, such an intense fire heat, a rapid spread, usually means there is very likely some form of accelerant of some description yet to be determined."
Dick Holmes, of Lincolnshire Police, said the five men had been killed "pretty much outright".
He described the sixth man as having been "very seriously injured".
The industrial estate was close to the town centre and made up of between 12 and 15 units that were "relatively small", he said.
Boston councillor Peter Bedford described the incident as a "real shock".
"There is heavy industry in that industrial estate, there's a scrap yard, there's joinery works, it's a real mix."
Fellow councillor Mike Gilbert said: "I'm very anxious to find out exactly what's happened. It's a lot of people dead and a great tragedy."
Canon Robin Whitehead, Rector of Boston, said the incident was "extremely sad and tragic".
Broadfield Lane Industrial Estate includes a furniture store, a scrap yard, a tyre firm and a car maintenance business. The site is surrounded by houses and is close to Boston town centre.