just to clarify slightly, the airlines are responsible for ground security of the people boarding their aircraft.
So, although the work at the scanners and so forth was naturally contracted out, the responsibility for making sure that the people on the plane are there legitimately rests with United. Whether they needed to beef their intelligence up, and have better scanners, or use improved risk computer systems is all conjecture, but the buck does stop at United.
Doesn't make it right though..