I know they're there. I don't know why but they seem to like toying with me, just siting there and watching me... Occasionally they'll actually do something and I'll wake up all cut up and bruised; I actually have a physical brand on my shoulder from one of those times, a nice big diamond.
Of course, since they have the capability to come all the way here we are beneath them, so I think we'll be all right, killing us is a waste of energy. They would much rather play conspiracy and mind games with us; they've already taken the mineral resources they want from us so we're not good for anything else to them.
I'm certain the government has access to some of their technology, after all they've recovered plenty of it. The government won't release it though, no matter how much it would improve our lives, because then no one could make money on any of the intermediate tech generations.
Have you ever looked at ancient alien theory? It's quite interesting.