Silly conspiracy theorys
If there was which there isn't any conspiracys regarding stuff like this you would probobally know about it. Our government agencys wouldn't be able to hold secrets like that for this long.
More likely we crashed a stealth bomber prototype or something at roswel, or a high altitude spy ballon, ( that was cold war times ) and the media came up w/ aliens that was the governments chance to completely divert attention from what was really going on. You've got to remeber what we have in our military, and what is developed at the moment is 10 years diffrence.