As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
EN ROUTE DEN-IAD, CREW RECEIVED EICAS MESSAGE PACK MODE L AND REPORT OF SMOKE IN CABIN. NR 1 AIR CONDITIONING PACK AUTO SHUTDOWN AND SMOKE DISSIPATED. AIRCRAFT RETURNED TO DEN. *S/D* PACK MODE L MESSAGE WAS DUE TO LT AIR CYCLE MACHINE SEIZING. AS THE ACM SEIZED, IT EMITTED A HAZE THAT MAY BE CONCEIVED AS SMOKE. THE LT ACM WAS REPLACED. THERE WAS NO FURTHER PACK MESSAGE OR SMOKE INCIDENT AFTER LT ACM REPLACEMENT.