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Investigators are looking at whether controllers' confusion delayed first responders as the plane was on its final approach, 9NEWS Aviation Analyst Greg Feith, a former NTSB investigator, said.

Only one of the 21 people on board the plane was taken to the hospital after the incident.

Tuesday morning around 8:30, United Express Flight 5912, operated by ExpressJet,declared an emergency landing during its final approach to DIA after being cleared to land, according to a control-tower recording provided by an FAA source. The emergency landing was because of smoke in the cockpit.

On the recording, a voice from the cockpit, either the co-pilot or pilot, is heard saying, "Emergency, smoke in the cockpit, roll trucks, please."

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

The voice responded, "5912."

The controller responds, after about 10 seconds, asking, "United 12, what's your position?"

After no response, more time elapses and the controller asks someone, "Did you hear that? I know that's BS. I know it is."

Another controller responds, "That's what?"

The first controller responds, "United 12. You know of United 12 anywhere?"

Feith says the controller may have been distracted, only hearing the last part of the flight number.

"So when they hear an oddball number," Feith said, "whether it's real or perceived, like in this case United 12. That's a bogus number so all of a sudden now you're spring-loaded towards, 'That's a phantom call.'"

Although rare, Feith says phantom or fake calls can originate from someone near the airport on the same frequency as the control tower.

"It's very complex because we've had these bogus calls in the past," Feith said.

"It sounded like somebody just (sic) on a handheld somewhere to me," the controller said later in the recording. "Did you hear that though?"

Another controller responds, "Yeah, I did."

"That didn't sound good," the first controller said.

"I know," responded the other controller.

The controllers then direct other traffic for another 30 seconds when the pilot makes another emergency call saying the plane had landed and was evacuating on the runway.

The controller tries to verify with the pilot, but after no response, he tells another pilot, "I apologize if you probably heard [something] there. That's not real. They're what we're hearing on the frequency."

Ultimately, nearly five minutes passed before the controller confirms to the pilot help is on the way, according to the recording.

According to his sources, Feith, said fire trucks did not deploy until after the plane had landed on the runway.

A passenger on flight 5912, Linda Irwin, says she saw smoke in the cabin during the landing, and said the pilot and co-pilot landed the plane extremely well, considering snowy conditions and smoke in the cockpit.

She also says the flight attendant remained calm during the evacuation from the front of the plane.

Irwin learned of the control tower recordings Thursday.

"One would hope that with all of the investigations that go on after any incident, one would hope that those communications would be checked out because when there are lives at stake," she said. "You don't want to make assumptions about what's real, what's not real. What's serious and what's not serious, you want to go with the worst case scenario and make sure you've addressed it."

Feith says NTSB investigators are looking at whether the Embraer 145 regional jet's right engine failed during final approach.

An FAA report says firefighters extinguished a fire in the instrument panel.

UPDATE: An NTSB spokesman says the Safety Board has not launched an "official" investigation into the incident.

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