When it comes information about extraterrestrials and covering up, all remember Roswell, however, there is probably even older case.
There are many people who do not believe in governments and in their information about extraterrestrials. Honestly at this time noone can believe in governments. In cover-up operations is invested considerable much time and effort and there are various secret protocols of which probably we do not even know, and all that just to get people away from the truth, so that those who believe in that starting to think that they are fools, and that do not need to believe their beliefs.
All people think immediately about Roswell, but apparently there's an older case. There are stories that in year 1937. during the Second World War in Germany were crashed alien spacecraft which was taken over by army. Apparently they trying whole time duplicate the alien technology and it is not known where the research took them. People said that the Germans were working on technology that bends space and time, but they were constantly faced with obstacles and they were not able to accomplish this technology. It is assumed that at least one invention can be directly attributed to copying of extraterrestrials technology, and that is the Horten brothers aircraft. Their aircraft was the first spacecraft with no tail and with jet drive, and also the first "invisible" aircraft.
In this case, government is still actively involved in covering up and hiding the truth, so that we have only stories.