Gravity control propulsion research had been the subject of widely published UFO literature. The documented testimonies of whistleblowers edited by Dr. Steven Greer, Director of the Disclosure Project ; anecdotes and schematics by Mark McCandlish and Milton William Cooper ; and the reports by Philip J. Corso, David Darlington, and Donald Keyhoe, famous UFO researcher, have suggested incorporation of reverse engineering of recovered extraterrestrial vehicles with the anti-gravity propulsion projects had enabled them to continue beyond 1973 to successfully manufacture antigravity vehicles. Branches of the military and defense agencies have denied and refuted such claims.