As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD - FLIGHT ATTENDANTS REPORTED SMOKE IN CABIN AND COCKPIT CREW REPORTED ACRID FUMES. EMERGENCY DECLARED FLIGHT RETURNED ORD LANDED WITHOUT INCIDENT. AIRCRAFT REMOVED FROM SERVICE. ACCOMPLISHED PACK BURN. NO ADDITIONAL ODORS NOTED. SYSTEM GROUND CHECK NORMAL OPERATION.