The explosion is believed to have been caused by an argonite cylinder used in a fire suppression system which exploded inside the building.
Group commander of Hertfordshire Fire and Rescue Service, Jon Smith, said a cylinder of the inert gas depressurised, acting like a missile and setting off other cylinders.
He said: "Each cylinder weighs about 50kg and is about 5ft tall, they've made a real mess of the inside of the building, they've taken down walls and floors.
"The external structure is intact, but 50% of the internal structure has been affected, walls are down, floors are up and walls knocked out.
"These cylinders have become missiles and fired around randomly."
(From:-
BBC NEWS | England | Beds/Bucks/Herts | Explosion man in critical state )
For some reason the valve was sheared off one of six 47kg 'argonite' cylinders being installed for fire protection purposes while the cylinders were being moved. The cylinder became a 'missile' and in flying around the building knocked the valves off the other cylinders which also took off, some ending up to 40m away from their original position.
(From:-
FireNet Forums / "Argonite" cylinder explosion at Welwyn Garden City )