That explosion started at the bottom of the building. Look at the burn marks on the building at the left. The blast blew a little debris out into the street, this hollowed out the bottom floors and then the debris fell down from above and piled up right in the middle. Look at the trees stripped of branches directly out front, the one to the left has full foliage. This rules out a bomb dropped from the air (although possibly a delayed fuze but that would have been stopped by the concrete about halfway down.) It also rules out a missile because they don't have missiles this big. The scorching looks like ANFO?