The Aspen (Colo.) Daily News said Monday United Express Flight 5940 was forced to return to Aspen/Pitkin County Airport Sunday and the aircraft was evacuated as a precaution.
SkyWest spokeswoman Marissa Snow, whose airline operated Sunday's flight, said 65 passengers were on board the Canadair Regional Jet 700 Series aircraft at the time of the incident. None of the passengers was hurt.
Bob Albright, a guest services desk volunteer at the Aspen airport, said the smoke that filled part of the aircraft's passenger cabin had a sulfuric smell.
Snow said while smoke did enter the aircraft's cabin, it did not enter the airplane's cockpit despite initial reports to the contrary.
Aspen airport director Jim Elwood said the investigation into the incident prompted the closure of the airport for nearly 20 minutes.
Elwood told the Daily News it appears the smoke was the result of a malfunction involving the aircraft's air-conditioning system, adding no signs of a fire or fire conditions were found on the airplane.