AA Flight 431 was traveling from Miami to San Francisco, and was over Utah, when the crew diverted from their course to make the landing. They touched down at 11:10 AM Pacific time and were met by emergency crews who assisted with the evacuation of the plane. All 159 passengers and six crew, exited without incident or injury.
A spokesperson for American Airlines said that the emergency landing is standard procedure for pilots after smoke is detected on a plane, and that the crew was just acting properly to ensure the safety of all those on board.
At this time, it is unclear as to the cause of the smoke, but investigations by AA flight mechanics and the FAA are ongoing. In the meantime the company is working to re-book all the stranded passengers and get them back on their way to San Francisco.