For more than 40 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 extra-terrestrial intelligence.
This document, which was only "discovered" in 1993, also gives credence to the cover-up stories.
- Artefacts left at some point in time by these life forms might possibly be discovered by our space activities on the Moon, Mars or Venus. (p. 215)
- Anthropological files contain many examples of societies sure of their place in the universe, which have disintegrated when they had to associate with previously unfamiliar societies espousing different ideas and different life ways; others that survived such an experience usually did so by paying the price of changes in values and attitudes and behaviors. (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: www.enterprisemission.com/brooking.htm