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.
- 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: http://www.enterprisemissions.com/brooking.html, which has several scanned pages of the report.