Complex sentence: When U.S. forces entered the compound he was hiding beneath the bed (probably seriously wounded), once U.S. forces entered, he started shooting and according to the official report, soldiers had to return the fire.
Split into simpler sentences: When U.S. forces entered the compound he was hiding beneath the bed (probably seriously wounded). Once U.S. forces entered, he began shooting and according to the official report, soldiers had to return the fire.