What was found
ON 10/13/2018, COMPANY SCHEDULED FLIGHT FROM HOU-DAL-ELP. AIRCRAFT DEPARTED HOU AND TRAVELED TO DAL WITH NO ISSUES. AT APPROXIMATELY 0200 CST, AIRCRAFT DEPARTED DAL ENROUTE TO ELP. AROUND 20 MINUTES INTO THE FLIGHT AT OR AROUND FL380, CREW EXPERIENCED LEFT HAND ENGINE ROLLBACK TO AROUND FLIGHT IDLE (50%-60%). CREW CARRIED OUT CHECKLIST PROCEDURES AND RETURNED TO ORIGIN AIRPORT OF DAL WITHOUT EVENT. CAUSE OF ENGINE ISSUE REMAINS UNKNOWN. ENGINE IS A RENTAL.