Of course they need to investigate, however, if FedX was given the clearance to land, and the airline was given the clearance to takeoff by controller, it's not an airline or Pilot's fault. Looks like they will be going to the recordings to see who messed up on which clearance.
finance.yahoo.com

UPDATE 2-U.S. investigates aborted FedEx landing in Texas, two planes cleared for same runway
The Federal Aviation Administration and the U.S. National Transportation Safety Board are investigating an aborted landing in Austin, Texas, on Saturday morning by a FedEx cargo plane that had been set to land on a runway on which a Southwest Airlines jet was also cleared to depart from, the...