tarmac at the gate with the air conditioners running. and the air conditioners are located right underneath the fuel tank right here so they provide a large source of heat. we did some testing and found out that the temperatures were well over 100 degrees in most of the tank. >> the temperature rose quickly on the fuel and it is turning to vapor very, very rapidly and filling up the container with vapors, an explosive mixture of vapors. >> when you provide the conditions together and they introduce a spark in there from any source, it will ignite and burn rapidly. it will explode. >> in some manner there was a spark. an ignition source. the safety board looked at all types of ignition sources. we looked at bombs and missiles and determined neither was a possible scenario. so our strong suspicion is that