As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF, TOWER ADVISED FUEL VENTING FROM LEFT WING. AIR TURNBACK, MADE RETURN TO ILN. LOST 1000 LBS OF FUEL IN FLIGHT. EMERGENCY DECLARED BY TOWER. FOUND LEFT OVER THE WING FILL CAP POPPED. CAP FOUND TO BE NOT LOCKING. REMOVED AND REPLACED LEFT OVERWING FILL CAP IAW DC9 AMM 28-10-04-20.