As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DCA - FLT 559 - EN ROUTE DCA-ORD, AFTER STARTING ENGINES COCKPIT FILLED WITH SMOKE. SHUT DOWN ENGINES, RETURNED DCA GATE WITHOUT FURTHER INCIDENT. REPLACED RECIRCULATION FAN. PERFORMED PACK BURNOUT. NO SMOKE NOTED. SYSTEM GROUND CHECKED NORMAL OPERATION. (M)