a) The culprit is American, is building on the postal/anthrax paranoia, and has a non-descript agenda/beef with the government.
It doesn't make sense that someone would take their "best shot", probably after months of planning, and end up with an incoherent "middle finger" to the government as his/her calling card.
2) This is a foreigner -- possibly Muslim, possibly leftist or Neo-Nazi -- who cannot make his case in a concise manner except to express displeasure with the government by afflicting those in the "heartland".
Nothing should be taken at face value. Just because someone says, "I've lived here a while", doesn't mean they haven't lived here their whole life. Just because someone says the government takes care of "their" people, doesn't necessarily mean he isn't included in that group also.
What it boils down to is this: People who are evil are servants of the Prince of Deception. Any scenario is plausible -- homegrown or Islamic terrorism. My hope is that whoever is doing the forensic testing on the debris is checking for anthrax residue...
Let's see what happens over the next few days. I hope this isn't a diversion of some kind...