UFO? Mysterious object over Denver nearly caused a mid-air collision Monday evening

The Federal Aviation Administration is investigating a mystery in the sky.

A unidentified object flying over Denver nearly caused a mid-air collision Monday evening.

As far as investigators know the object did not show up on radar.

Investigators believe the object, whatever it is, could pose a serious safety hazard to planes.

Radio transmissions from LiveATC.net confirm a nervous-sounding pilot reported a strange object at 5:17 p.m. Monday.

The pilot is heard telling air traffic control: "A remote controlled aircraft, or what? Something just went by the other way ... About 20 to 30 seconds ago. It was like a large remote-controlled aircraft."

The corporate jet, a Cessna Citation 525 CJ1, was flying at 8,000 feet above sea level over Cherry Creek when the mystery object came close enough to make any pilot nervous.

"That's an issue because now we have something in controlled airspace that poses a danger," former NTSB Investigator and aviation analyst Greg Feith said.

Feith listened to the air traffic recordings and believes the object could be one of three things:

- A military or law enforcement drone.
- A remote controlled aircraft.
- A large bird.

"Was this an unmanned vehicle that was part of some sort of law enforcement operation? Was this somebody that had flown a large model aircraft inadvertently into the airspace? Or was it just a bird that caught the pilot's eye so he believed it was an aircraft but could have been a very large wing span bird," Feith said.

Any one of those things can be catastrophic if it collides with an airplane.

Three years ago, a bird strike took down a commercial airliner that managed to land safely in the Hudson River. All the passengers survived.

FAA spokesman Mike Fergus says investigators will talk to the pilot and look at other clues.

"The threat is there from a collision standpoint. We'll do as much as we can here to try to track back what time it was. Probably talk with some remote-control clubs, that type of thing," Fergus said.

The mission of investigators now is to identify that mystery object before another close call, or worse.