A mystery object, thought to be a military or law enforcement drone, flying in controlled airspace over Denver almost caused a catastrophic mid air crash with a commercial jet Monday.

The pilot of the Cessna jet radioed air traffic controllers to warn them that “A remote controlled aircraft” had flown past his plane far too close for comfort.

“Something just went by the other way … About 20 to 30 seconds ago. It was like a large remote-controlled aircraft.” the pilot said in the transmission that was captured on the live air traffic audio website liveatc.net.