Complex sentence: The attack was conducted by gunmen dressed as Salvadoran soldiers, and in total twelve people were killed: four United States Marines, two United States businessmen, a Guatemalan, a Chilean, and four Salvadorans.
Split into simpler sentences: The attack was conducted by gunmen dressed as Salvadoran soldiers, and in total twelve people were killed. The victims were four United States Marines, two United States businessmen, a Guatemalan, a Chilean, and four Salvadorans.