As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER DEPARTURE, THE FLIGHT ATTENDANT NOTIFIED THE CREW THAT THE LEFT HAND ENGINE INBOARD COWLING WAS OPEN. FLIGHT 4083 RETURNED TO IAD AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE INSPECTED AND SECURED THE COWLING IN ACCORDANCE WITH DORNIER MAINTENANCE MANUAL 72-00-00. THE AIRCRAFT WAS THEN RETURNED TO SERVICE.