DENVER—The Federal Aviation Administration is investigating after an air traffic controller was accused of ignoring a request for an emergency landing in Denver after a commercial airline pilot reported smoke in the cabin.

The controller thought it was a prank and dismissed the emergency call, according to recordings obtained by KUSA-TV ( http://on9news.tv/HkWYhP).

The United Express plane from Peoria, Ill., was evacuated Tuesday after landing at Denver International Airport. An FAA report said firefighters extinguished a fire in the instrument panel. The cause of the fire was under investigation.

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.

A transcript of the recording seems to indicate that the controller misunderstood the airliner’s flight number. The FAA says the pilots didn’t initially indicate their airline when talking to controllers.

“Although the pilots of ExpressJet Flight 5912 did not initially indicate their call sign when they contacted air traffic control to declare an emergency, the controllers were able to quickly identify and locate the plane with the help of ground surveillance equipment and immediately alert the fire department, whose first truck arrived shortly after the plane came to a full stop,” the FAA said in a statement Friday.

According to KUSA-TV, a voice from the cockpit, either the co-pilot or pilot, is heard saying, “Emergency, smoke in the cockpit, roll trucks please” as the plane was coming in for a landing.

A controller in the tower responds, asking, “Who was that?”

The voice responded “5912”—the flight number that air controllers were tracking.

After some confusion, the controller responds about 10 seconds later, asking: “United 12, what’s your position?”

After no response, more time elapsed before the controller says, “Did you hear that? I know that’s BS. I know it is.” Controllers said they were not aware of a United Flight 12.

The National Transportation Safety Board said the investigation has been turned over to the FAA.

An FAA spokeswoman wouldn’t comment on the controller’s statements on the recording.

The airline refused comment.

Aviation analysts and a former pilot said Friday the first response from a controller should be to clear the decks of other aircraft after a pilot declares an emergency and treat the call as authentic.

Sid McGuirk, who teaches traffic management at Embry-Riddle Aeronautical University in Daytona Beach, Fla., said pilots are in control when there is an emergency and failure by controllers to respond would be a major violation of procedure that could result in discipline or retraining.

“They have to assume it’s a real emergency, whether it’s a Cessna or a 747 jumbo jet. If it later turns out to be a spoof, it’s a federal crime,” McGuirk said.

William Voss, president of the Flight Safety Foundation in Alexandria, Va., said pilots who declare an emergency are busy dealing with the problem, and it’s easy to create confusion.

He said radios with airline frequencies are easily available and have been misused to report false emergencies.

The union representing air traffic controllers did not return phone calls seeking reaction.

Patrick Shanahan, a retired controller who worked at one of the world’s busiest air traffic control facilities in New York, said he has never heard of anyone from the public or another controller pretending to be an airline pilot with an emergency.

Even if that were the case, “it’s beyond belief to me that a controller would just let that go and think someone was playing with him,” said Shanahan, a former union official.

Katie Pribyl, a former United Express pilot, said she’d never heard of an air traffic controller not taking a request for an emergency landing seriously, but she said she also never heard of someone faking an emergency landing request.

“In my experience as a regional jet pilot, controllers normally took reported malfunctions very seriously,” Pribyl said. “One of their first questions is usually, are you declaring an emergency?”

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Associated Press writer Joan Lowy also contributed to this report.

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Information from: KUSA-TV, http://www.9news.com