A pilot or co-pilot on a United Express flight from Peoria, Ill., reported smoke in the cockpit and requested fire trucks to meet the plane on the runway.
An air-traffic controller in Denver apparently misunderstood the flight's call letters and dismissed the emergency call as a prank.
The plane was evacuated Tuesday after landing at Denver International Airport. An FAA report said firefighters extinguished a fire in the instrument panel.
Controllers apparently realized the mistake when the pilot made another emergency call saying the plane had already landed and was evacuating on the runway. It was only then that fire trucks responded.
One of the 21 passengers was taken to the hospital. The name of the patient and condition were not available.
The National Transportation Safety Board said the investigation has been turned over to the FAA. The airline refused comment.
The cause of the fire was under investigation.
The Associated Press contributed to this report.