As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AIR TURN BACK, AIRCRAFT CABIN FILLED WITH SMOKE, RETURN TO ST-LOUIS. AIRCRAFT DEPARTED WITH THE APU UNDER MEL DUE TO LOW OIL QUANTITY & APU SHUTDOWN (APU FAILURE) OIL FROM THE APU WENT IN THE LT PACK CAUSING THE SMOKE. REPLACED THE APU AND THE PACK.