Brookings Report

For approximately 60 years, the "Proposed Studies on the Implications of Peaceful Space Activities for Human Affairs," or the "Brookings Report" as it is usually called, determined the policy of NASA with regard to extraterrestrial intelligence.

This document, which dates back to 1961 but was only "discovered" in 1993, also gives credence to the UFO cover-up stories.

Some quotes:

photocopy Brookings Report, p. 215

On p. 216 recommendations are made for more studies. The suggestion that "such information (...) be withheld from the public" is also explicitly made.

For more information:, which has several scanned pages of the report.