Did Aliens build the UFOs or did We?

This has been going around my mind for a while now, and I am still on the Edge of knowing which of both to believe.
There are many facts that point towards Us building them, yet more facts pointing towards an extra-terrestrial form of Intelligence being the cause for the Phenomenon hauting people all around the world for more than 50 years now.
One of the biggest hints to the answer of this question, is the theory, of the nazis being highly involved in Anti-gravity research as well as field propulsion systems and others. I know that this theory is not very respected, it still seems to be more credible than the claims of little gray or green men coming here to kidnap people. Do you really think that they would need to experiment on us if they were capable of builing Aircraft, able of reaching our planet? There are no Planets known to scientists, that bear life on them, so that would mean that they traveled a longer distance, than we can look out there. Them journeying here would prove that they are lightyears ahead of us in technologically, so they would probably already know more about Humans than we humans do. There would be no reason forexperiments, or kidnapping. Why would Area 51 be worldfamous for sightings or circular aircraft and socalled "UFOs" if there wasn't any truth to those numerous claims? But on the other side Why would this be such a controversial subject if there were definate evidence for either of both claims?