A police station had to be evacuated after seven people were injured in a blast at a Scotland Yard firing range armoury.
One construction worker was airlifted to hospital with serious injuries after the blaze ripped through the basement of the building in Croydon, south London.
Officers ruled out terror-related causes and said there was no weaponry in the room where the explosion was sparked. More than 100 police staff and officers and 14 prisoners were moved to neighbouring South Norwood police station during the alert.
One victim was said to be in a stable condition with burn injuries while a police officer, two police staff and two builders were treated for smoke inhalation. The firing range used by police had already been decommissioned for upgrade work.
Chief Inspector Caroline Trevithick said: "There was an industrial accident in the basement.
"We have a total of seven casualties and five have gone to hospital. One had serious injuries, and one is in a stable condition. A couple of police staff and one police officer were treated for smoke inhalation.
"We don't know what caused the explosion, but we believe it was in the armoury. There was nothing in there as it was in clearance while work was going on."
The Health and Safety Executive is carrying out investigations at the closed Croydon site.
A spokeswoman for Vinci Construction UK, which employs the builders, said: "Vinci Construction UK is gravely concerned about the welfare and safety of the construction workers and others affected by this incident and has been working with all emergency services throughout the day to secure the scene of the incident. We are working with the HSE in their ongoing investigation to understand the cause of the explosion."
Of those injured, one remained in hospital in a serious condition on Tuesday night and another remained in hospital in a stable condition.