The Pentagon, at the direction of Congress, a decade ago quietly set up a multimillion-dollar program to investigate what are popularly known as unidentified flying objects—UFOs.

The "unidentified aerial phenomena" claimed to have been seen by pilots and other military personnel appeared vastly more advanced than those in American or foreign arsenals. In some cases they maneuvered so unusually and so fast that they seemed to defy the laws of physics, according to multiple sources directly involved in or briefed on the effort and a review of unclassified Defense Department and congressional documents.

The Advanced Aviation Threat Identification Program, whose existence was not classified but operated with the knowledge of an extremely limited number of officials, was the brainchild of then-Senate Majority Leader Harry Reid (D-Nev.), who first secured the appropriation to begin the program in 2009 with the support of the late Senators Daniel Inouye (D-Hawaii) and Ted Stevens (R-Alaska), two World War II veterans who were similarly concerned about the potential national security implications, the sources involved in the effort said.