According to the New England Journal of Medicine, the most likely source of Stevens' exposure was the so-called "J-Lo" letter. Published reports are sketchy and somewhat contradictory, but suggest this package was received about a week before 9/11, set aside as "kook mail" until later, and Stevens was exposed to the contents on 9/19. Even if the reports of a delay between the package's arrival and Steven's exposure to the contents are accurate, it is still not clear whether the package was opened on arrival, or whether it was simply set aside in the mail room unopened, until it piqued somebody's interest.
Wasn't there a mail-room clerk/intern of middle eastern descent that was initially under suspicion?