The fire was thought to have been caused by a gas explosion.

explosion 是名词，来自动词 explode（爆炸），指"爆炸"或"爆发"的现象。 gas explosion 指"气体爆炸"，即由于可燃气体（如煤气）与空气混合后被点燃而发生的剧烈爆炸现象。