He said no conclusions had been drawn, but played down any “conceivable threat” to the United States.

Later that year, however, H. Marshall Chadwell, the assistant director of scientific intelligence for the C.I.A., concluded in a memo to the C.I.A. director, Walter Bedell Smith, that “sightings of unexplained objects at great altitudes and traveling at high speeds in the vicinity of major U.S. defense installations are of such nature that they are not attributable to natural phenomena or known types of aerial vehicles.”

By 1953, authorities were concerned that communication channels were becoming dangerously clogged by hundreds of U.F.O. reports. Even false alarms could be perilous, defense agencies worried, since the Soviets might take advantage of the situation by simulating or staging a U.F.O. wave and then attack.

Documents show the C.I.A. then devised a plan for a “national policy,” as to “what should be told the public regarding the phenomenon, in order to minimize risk of panic.”

After a closed-door session with a scientific advisory panel chaired by H.P. Robertson from the California Institute of Technology, the C.I.A. issued a secret report recommending a broad educational program for all intelligence agencies, with the aim of “training and debunking.”

Training meant more public education on how to identify known objects in the sky. “The use of true cases showing first the ‘mystery’ and then the ‘explanation’ would be forceful,” the report said. Debunking “would be accomplished by mass media such as television, motion pictures, and popular articles.”