Complex sentence: A subsequent Naval Board of Inquiry never determined the exact cause of the disaster but concluded that the initial explosion was caused when a mortar round aboard detonated during an unloading operation because it was either dropped or went off when gasoline vapors ignited.
Split into simpler sentences: A subsequent Naval Board of Inquiry never determined the exact cause of the disaster. But it concluded the initial explosion was caused when a mortar round aboard detonated during an unloading operation because it was either dropped or went off when gasoline vapors ignited.