WHILE TAXIING IN FROM A LANDING THE LEFT ENGINE CAUGHT ON FIRE. SUBSEQUENT EXAMINATION REVEALED THAT THE FUEL LINE FROM THE FUEL PUMP TO THE FUEL CONTROLLER WAS CROSS THREADED AT THE FITTING AT THE FUEL CONTROLLER. TOTAL MATING OF THE FITTING SURFACES DID NOT OCCUR. MOST RECENT MAINTENANCE ON THE FUEL LINE COULD NOT BE DETERMINED FROM MAINTENANCE RECORDS.