“2. The ricin was sent from Canada. So either an American crossed the border to mail it from there, or it was an international attack.”
I predict sometime in the next few months we’ll have reports of a Canadian invasion on the 6pm news.
Your sarcasm aside, Canada's immigration system is more lax than ours (hard as that is to imagine), so it isn't outside the realm of possibility that a non-Canadian and non-American person did this.