Credit: ABC News

ABC News' Mark Greenblatt and Olivia Katrandjian report:

Safety experts are stunned after a passenger plane filled with smoke while trying to land at a snowy Denver International Airport, but  the air traffic controller dismissed the call for help from the pilot as "B.S."

The April 3 flight from Peoria, Ill., was scheduled to land in Denver. Everything was going according to plan until smoke filled the plane.  The pilot of the United plane called the control tower to request an emergency landing.

"This is 5912.  Emergency, smoke in cockpit, roll trucks, please," said the pilot.

But the control tower did not know who was calling.

"And who was that?" the controller replied.

"5912," the pilot said, but the control tower apparently heard "United 12."

"United 12, what's your position?" the controller asked.

The pilot did not respond, and the controller decided it was a hoax.

"Did you hear that?" the controller asked another controller. "I know that's BS. I know it is."

"United 12. You, you know of United 12 anywhere?" he added.

"And, ah, I apologize. If you probably heard there, that's not real what we're hearing on the frequency," the controller announced to his coworkers.

The plane prepared for an emergency landing believing that fire crews had been dispatched and were ready to assist.

When the plane landed, the pilots called the control tower again in a panic.

"We're on the runway!  We've been evacuating! We've been evacuating!  34 right!" the pilot screamed.

But even with the plane on the ground, the controllers still didn't believe the emergency was real.

"12 verify that wasn't you," said the controller.

Prank calls to control towers are not that uncommon and the controllers may have thought this was one.

But longtime pilot and aviation expert Kevin Hiatt says lives were hanging in the balance at a time when seconds mattered.

"This is a very serious situation and could have had a very disastrous result," said Hiatt.

"I think it's very troubling. An air traffic controller should never assume or jump to a conclusion at any time," Hiatt added.

Aviation experts say that the pilot was wrong in initially identifying himself as only as "5912." He should have identified himself as "United Airlines 5912."

Luckily the pilots landed and evacuated the plane safely,  without help from the control tower.

The FAA has yet to comment on the incident.