What was found
A STRONG SMELL OF JET FUEL CIRCULATED ABOUT THE CABIN AND COCKPIT. CREW SWITCHED FROM APU SUPPLIED AIR TO ENGINE AIR AND FUMES REMAINED/REOCCURRED. COCKPIT CREW DONNED OXYGEN MASKS FOR DURATION OF FLIGHT. CREW NOTED THAT THE FUMES WERE STRONG ENOUGH TO CAUSE THEM HEADACHES. LANDING IN DTW WAS NORMAL AND PASSENGERS DEPLANED NORMALLY. NO EVIDENCE OF ANY FUEL LEAKING FROM EITHER POWERPLANT, APU, FUEL TANKS, OR FUEL LINES. THE AIRCRAFT RETURNED TO SERVICE.