"Makes me think it may have been an activation card to active other silent cells." trying to follow the logic, if it was an "activation card" as you call it for other cells, WHY would they (the shooter who left it) want it kept quiet ???
What better way to ensure that something is said than to tell someone not to tell it. Seems to have worked doesn't it. Card says (reported by Fox today) "Do not let the News know about the card".