A man died and six people were injured when a gas canister exploded at a warehouse.
Emergency crews were called to the site at the Mundells, Welwyn Garden City, Hertfordshire, at 1500 GMT.
Six people, one with life-threatening injuries, were taken to the Queen Elizabeth II Hospital in the city.
Police later said that one man had died in the explosion. His family has been notified and his identity is expected to be released on Thursday.
Another injured man did not need hospital treatment.
Police and fire crews are searching the building.
A Hertfordshire Fire Service spokesman said 10 crews had tackled the fire and there were gas canisters inside.
'Building intact'
Hertfordshire Police said one of the people taken to hospital was seriously injured and four had minor injuries.
A police spokeswoman said "The building itself has sustained some damage but remains largely intact which enables the search to be continued by police officers.
"The Health and Safety Executive are aware of the incident and are monitoring the situation."
She added the operation had been scaled back but emergency services would remain at the scene overnight carrying out forensic tests and a building search.
A spokesman for the Health and Safety Executive said the site had been run by pharmaceutical giant GlaxoSmithKline, but was being renovated or converted.
Nobody from the company was immediately available for comment.
Roads later opened around the area and traffic was moving. Police said businesses and work would return to normal on Thursday morning.
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