A police station was evacuated today 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 this morning.
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 was carrying out investigations at the closed Croydon site tonight.
Steven Keable, 33, who works opposite the building at Cruse and Burke accountancy firm, said: "There was a gentleman on a stretcher who had obviously suffered burns.
"There were hundred of people involved. The area was cordoned off and everyone was evacuated out of the buildings.
"We were watching but we weren't evacuated. It was a bit worrying really."
Emergency services were called at 11.25am.
A London Fire Brigade spokesman said six engines and 30 firefighters were sent to the scene in Park Lane.