Complex sentence: Moussaoui had enrolled for training in a 747-400 simulator at the school on August 13 but his behavior during instruction alarmed Prevost, who did not believe that Moussaoui was just a wealthy individual interested in flying jumbo jets.
Split into simpler sentences: Moussaoui had enrolled for training in a 747-400 simulator at the school on August 13. Moussaoui's behavior during his flight instruction alarmed Prevost, who did not believe that Moussaoui was just a wealthy individual interested in flying jumbo jets.