A couple of thoughts -- forgive me if I'm repeating -- I didn't read through all the posts --
(MidEast angle) Number does not belong to the killer but to an embassy somehwere in the world where there is no U.S presence. Accomplices make their demands while they're operative moves on in the field.
My guess is that the lid is being kept tight on this perp out of fear that any confirmation of a terror source may be a trigger for other sleeper cells to activate...
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the lid is being kept tight on this perp out of fear that any confirmation of a terror source may be a trigger for other sleeper cells... That's a horrifying thought, but given the scale of the reaction going on now (closing of all of those schools, etc.) it's certainly not impossible.
Granted, it would be terrible even if it were a lone psycho and even if only one more person were killed. But I think it's probably in a whole different league.