I take exception with the idea that someone could pull it off when a story breaks at 6:00 PM (mail shuts off earlier then that...and these things would have to be mailed from the east if the address was in the east to get there next day)...the person would have to get the idea (upon the stpry breaking...not first hearing about it) choose a target, get the supplies and have it to the post office in one hour. Multiply this happening in several different locations. Cannot be a lone nutbag....next to impossible.
But if they've studied the situation and have an incling of the incubation period they could pre-mail the second batch (that's what I was trying to get at but didn't do a good job as you can see). Also there's the possibility of the two waves not being related, of idiots (similar to the ones that were calling in al those bomb threats the week of the attack) mailing talcum powder to local (for them) places, that would give that next day thing, especially if your sending to big places that get multiple deliveries.
One of the dangers with this one, especially something this low rent to fake, is assuming it's all coming from the same source. There are way too many idiots in this country to assume no one has gotten the bright idea to copy these "attacks" with harmless powder. This is something almost every household in the country could fake, just need some sort of white powder and envelop and a stamp and you're ready to rock.