BURLINGTON, Wis. (AP) – Federal investigators are beginning their work to find out what caused the fatal crash of a small plane in southeastern Wisconsin.         Two people were killed when the single-engine Grumman crashed in a cornfield while attempting a landing Sunday afternoon at the Burlington Airport.        Federal Aviation Administration records show the plane is registered to a man from Antioch, Ill.  FAA spokesman Lynn Lunsford says it’s not immediately clear where the flight originated.  The victims have not yet been identified.