Extraordinary pictures have emerged of a triangular aircraft soaring over Amarillo in Texas.
Aviation experts believe it could be a classified military craft - of a shape not seen since 1956 - used for "a high-precision stealth attack".
"Classified programmes have been exposed in all sorts of ways, for example, the A-12 Blackbird was disclosed under a degree of pressure," Bill Sweetman of Aviation Week said.
"It's not merely logical to expect that numerous classified aircraft programmes exist. It's almost a necessity."
The Aviation Week team believe the aircraft is real.
Sweetman added: "The photos tell us more about what the mysterious stranger isn't than what it is.
"The size is very hard to determine, for example, although the image size at contrailing height suggests that it is bigger than an X-47B. However, the basic shape - while it resembles Boeing's Blended Wing Body studies or the Swift Killer Bee/Northrop Grumman Bat unmanned air system - is different from anything known to have flown at full size, lacking the notched trailing edge of Northrop Grumman's full-size designs."
The aircraft is believed to have been accompanied by two others. Photographer Steve Douglass also picked up voice traffic - meaning the aircraft was piloted.
Sweetman said to look at gaps in the United States Air Force "line-up" to find answers to the mysterious sighting.
"One obvious example is high-precision stealth attack. The B-2 and F-22 have the ability to drop GPS-inertial weapons on coordinates generated or updated by radar, but that's not the same as electro-optical targeting and laser guidance, which seemingly went away with the retirement of the F-117 six years ago: that technology gives you strike damage assessment as well as greater accuracy," he said.
"The USAF has also talked about 'penetrating, stand-in electronic attack' as an enabler for other strike systems - and talked in the same way about penetrating intelligence, surveillance and reconnaissance, even when the Northrop Grumman RQ-180 was under development to perform that mission."