Of course there are more than just those 19. It's not MY bubble you're bursting. It's yours - that Atta was the culprit.
If you want to suggest that Atta mixed all this up and set up the letters before leaving for Massachusetts then it means some accomplice had it sitting in their house for at least ten days before deciding to mail it. Maybe it was one of those who didn't know they were being sacrificed like some of the hijackers reportedly didn't "need to know".
Which then leads back to my original point - why aren't we looking at New Jersey hospitals?