It seems more likely that rumors of aliens is obfuscation of the military by the military to avoid rumors about secret programs. Non-human biologics are easy to fake and photograph and might pass if no close or detailed examination is made.
No need to fake 'non-human biologics' when a bird can poop on the wreckage and viola "'Non-human biologics' were found on crashed craft." Vague wording can be used to justify almost anything for people who are predisposed to believe it.
