United Express Flight 5912, operated by ExpressJet, made an emergency landing at DIA just after 8:30 a.m. because of smoke in the cockpit.

FAA spokesman Mike Fergus says there is an active investigation into the incident, but would not elaborate or speculate on the scope of controllers' interviews or whether any air traffic controllers could face discipline.

The FAA is in charge of air traffic controllers across the country.

The plane's pilot made a mayday call while on final approach to DIA, asking for fire trucks to meet the plane at the runway because of smoke, but recordings show air traffic controllers did not understand the call.

Fire trucks were not dispatched by air traffic controllers until after the plane was on the ground, nearly five minutes after the pilot requested the trucks, according to 9NEWS Aviation Analyst Greg Feith.

A controller is heard on the recordings telling another pilot, the voice on the frequencies was not real. He also conversed with another controller, saying he thought the call was "BS," and maybe someone cutting into the air traffic control tower's frequency.

NTSB spokesman Peter Knudson said Friday morning the safety board would not launch an official investigation into the incident. The NTSB routinely investigates airplane accidents, though this incident, and the apparent confusion did not rise to the level of an "official investigation," Knudson said.

An FAA spokesman said Thursday, 21 people were on board the United Express Flight from Peoria, Ill. Only one person was taken to the hospital.

The FAA released the following statement about the incident:

"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."

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 National Transportation Safety Board said the investigation has been turned over to the FAA, which said it would comment later Friday. The airline refused comment.

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," Shanahan, a former union official, said.

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?"