The article says he was interviewed by the Milwaukee police on Sept. 18, apparently in Milwaukee. That is the postmark date of the letters to the NY Post and Tom Brokaw, postmarked Trenton, NJ. These letters were not mailed from Milwaukee (in addition to the postmark evidence, note that there's no trail of anthrax spores or anthrax infections in Milwaukee).
Sept. 18 was a Tuesday, so the letters were presumably mailed either on Tuesday itself or after the last pick-up time on Monday. Unless this scientist had just returned to Milwaukee from out of town either on Sept. 18 or late on Sept. 17, this just doesn't make sense.