I agree. I believe the longer this goes on without the killer being caught, the more likely it is MR-related terror and the more likely it involves one or at most two or three people. That is, the killer is his own autonomous cell or a member of a very small and internally-loyal autonomous cell. I suspect telephone calls are not being placed and email is not being exchanged between the killer(s) and home base, because our spooks would almost certainly have zeroed in on them--particularly once the message traffic leaves the US.
The killer seems to be demonstrating blood-chilling discipline and caution. I tend to believe he is not a home-grown psychotic--but a Muslim zealot with a rifle.