Whether it's foreign terrorists, domestic terrorists or both, the bottom line is: this is terrorism.
Two possibilities hit me: 1:) the letter was planted later by somebody else hoping to cash in, which is why police didn't discover it at the scene until a tipline call led them back to the scene of the shooting.
2:) It's an extortion scheme, the success of which depends upon the effectiveness of the terror tactics, a la the rash of plane hijackings in the '70's which involved monetary demands, or the much more recent tactics of Abu Sayyef in the Phillipines.
I agree with you...either way, there WILL be more of of these incidents, and the media highlighting our soft underbelly will just embolden the psychos into trying more often and harder.