Investigation
The investigation of the NTSB revealed that several persons had reported an alleged gas leak in the building in the days leading up to the explosion. The store had no gas supply, so another nearby gas line looked like the culprit. It was discovered that a gas pipe which carried the heavier-than-air propane gas was broken. A few years earlier, a water main was installed below, which bent the pipe in the process. When the pipe had been installed, it was already in tight bend, adding to its stress levels. The addition of the water main caused it to break.
The deadly gas was able to get into the shop basement from migrating around and over pipes, causing the bad smell. However the biggest problem was that the gas company technicians were unable to detect the gas before the blast. Investigators discovered that holes used to detect gas below were only 46 cm (1.5 feet) deep when the gas was about 4 feet down, therefore, there was no way it could be detected in this manner. The most crucial error came in the basement inspection. The technician was supposed to turn his equipment on in fresh air before entering the building, but he instead turned it on once inside the building, thus detecting no gas.
The ignition source was an air conditioning switch with heated wiring, causing the whole building to be stripped to its skeleton.
The San Juan Gas Company vehemently denied responsibility and suggested that it could have been caused by sewer gas instead. However, the lighter-than-air sewer gas collects in the ceiling, while propane collects on the floor. The explosion showed that stored shoes were tossed up, meaning that the fuel had to be at ground level.
Read more about this topic: Humberto Vidal Explosion