Occam's razor folks. They are finding anthrax in the office. If he was the source, what are the chances it spread to the office but not his home, it spread to co-workers, but not to his wife and kids.
Well sense all the cases so far worked at a "newspaper", it could be ,if these people all worked the field, they may have gone to a common site doing a story and came in contact with anthrax there.
And considering a large troop of "Osama A hole's" were based in the local area it could have been an easy local story and they stumble on to a terrorists anthrax stores with out knowing it.
Anthrax is not contagious. Therefore, it doesn't spread to anyone. It's all about exposure to spores. So, the chances are 100% that if the exposure were to spores at the office, his coworkers would get it but not his family.