As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMBOUT AT MID 20 THOUSANDS ALTITUDE, L ENG FIRE WARNING ACTIVATED. CREW REDUCED ENGINE POWER TO IDLE AND WARNING EXTINGUISHED. CREW RETURNED FOR LANDING WITH NO FURTHER ABNORMAL INDICATIONS. CAUSE OF FIRE WARNING IS UNKNOWN AT THIS TIME.