As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF LOW OIL PRESSURE WARNING LIGHT CAME ON. CREW MONITORED AND MANUALLY SHUT DOWN NR 1 ENGINE. CREW ELECTED TO RETURN TO BASE. CREW FOUND OIL FILL CAP ON LEFT ENGINE NOT SECURED CORRECTLY. THE ENGINE OIL HAD VENTED OUT THE OIL FILLER.