Much like the Manhattan Project, many of the people working on it had no idea what they were actually involved in due to its compartmentalized nature.
If you were to believe the people involved with the subject, these would be the steps to maintain such secrecy, in no particular order:
- Compartmentalize and stovepipe the research.
- Deny and dismiss any reports.
- Provide prosaic explanations to all reports, even if they might be bogus.
- Discredit individuals who come forward.
- Stigmatize the issue.
- Utilize the mainstream media to assist with steps 2 through 4.
The initial reports of Roswell stated that debris from a flying saucer had been recovered. However, the narrative later changed to it being a weather balloon. This switch serves as an example of a cover-up.
John Greenewald from The Black Vault has expressed his view and essentially demonstrated through FOIA requests that there is a cover-up of some kind, though he hasn't expressed what he believes is being covered up. Whether this cover-up involves black projects, a psyop, or non-human technology, we ultimately require more solid evidence to determine this with certainty. The amount of high ranking credible military and government personnel coming forward saying "Non-Human Intelligence" does cause people to lean a certain way, however.