[FairOpinion]

There are no details given.

My guess, and it is strictly a guess, is that after he died they went through his possessions, and opened the suitcase and found anthrax, or at least something that tested positive for anthrax in preliminary tests, so you put two and two together: the guy died of anthrax, and they find, what is likely to be anthrax in his suitcase, and maybe they know something about his background, that he is not really associated with terrorists, you can conclude what the articles have been saying that he was a carrier, and got curious. If he knew it was anthrax, he would not have exposed himself knowingly.

Also there may be more to this, than we are told, someone posted an article from back in February, that the Feds were watching this particular ship.