The spy aircraft, the NASA 844, has been absent from its pad since at least May 17, according to Aviation Week.

Sources told the publication that the plane was moved into a hangar earlier this year for an inspection, though NASA has not described what that inspection involves, why it is taking place, or how long it will last.

The Serial 61-7980 was the last SR-71 built and the last to fly, completing its final flight in 1999 after the Air Force had already retired the type and NASA had retained a few surviving airframes for high-speed research.

Lockheed Martin produced 32 of the reconnaissance aircraft, 12 of which were lost in accidents and none to enemy action.

Questions about the disappearance of the SR-71 come a week after NASA Administrator Jared Isaacman discussed plans for a new high-altitude, high-speed X-plane and displayed images of silhouette that closely resembled a Blackbird at annual the All-In Summit in Los Angeles. 

"In the service of the first A [aeronautics] in NASA, we are rebuilding our X-plane fleet. The X-59 is researching quiet supersonic flight, but it's just the beginning," the NASA head told attendees.

"As NASA recommits to flight test and works alongside industry to push the boundaries of airframe and propulsion design, it will not be long before NASA once again is flying as high and as fast as we had in decades past, and then even more."

Asked whether the image was an SR-71, Isaacman quiped, "You sure that's a Blackbird?" and declined to elaborate.