The most likely possibilities appear to be numbers two and three, that this was either a test run or a botched attempt at infecting the population. The experts have been claiming for weeks that handling these bioagents isn't as trivial as fearmongers would suggest. It seems probable that such factors are at work here.

It is reasonable to guess that if two people have shown signs of contamination, those two people are the ones who handled the stuff. A receives the package/envelope, handles it while sorting, picks nose. B opens package gets heavy dose on hands and hence keyboard, enough is around for him to inhale it. Terrorists overestimated ease of distribution and had some misconceptions about risk of contagiousness - they may have thought mailing it to one guy would infect the whole workplace. Lack of positive tests on other locations/mail : xlow concentrations, things thrown in trash over past week.