The collapse of a 24 story wing of a newly occupied high-rise apartment in London last May is making experts take a long, hard retrospective look at the disaster in an inquiry that could have implications for prefab or component building in high-rise structures. Ronan Point, as the now famous building is called, was one of nine similar blocks under construction for the London Borough of Newham using the Danish Larsen and Nielsen system of prefabrication. The method, widely used throughout Western Europe and in Turkey and Hong Kong, employs precast factory made components. An interim letter put out by the inquiry in mid-August indicates that a gas explosion was the immediate cause. The explosion, thought to have originated on the 18th floor of the building, apparently blew out two load-bearing exterior wall panels, leaving the six floors above without support. These progressively fell, taking with them the apartments below. But the root of the failure was found to lie in the joints that connected the stacked floors and wall slabs, as it was observed that their design did not provide continuity connection to assure mutual interaction of the components under severe overload. The theory is that the explosion followed ignition of gas pockets accumulated at ceiling level. But experts agree that other factors, however remote- including ground settlement, accidental damage and other types of internal explosions-could likewise have caused the failure. In the words of one British engineer, Bernard Clark, speaking before the inquiry, "We should not have buildings such that, when there is an explosion, the whole lot comes down. This is where the weakness of the joint is shown up. If this building had been joined properly at the joints, it is quite possible that the ceiling and some internal walls may have cracked, but we would not have had a cascading collapse." Progressive collapse in frame building is virtually unknown, it was observed.