Plane's wing catches fire just before taking off

A Southwest Airlines flight from Detroit to St Louis was grounded after a passenger informed that one of the plane's wings was on fire just before takeoff. David Fiedor was sitting in the rear of the Flight 1863 when he heard a noise that didn't strike him as odd until he looked out the window and saw the wing had flames coming out from the bottom.
David quickly approached a flight attendant who then notified the pilots and the plane returned to the gate. "I heard a muffled thud similar to when the landing gear is engaging. Then I looked out of the window and saw the wing was on fire," David was quoted saying in the Daily Mail.
He added that the flight attendant initially did not notice the flames as the fire was intermittent.