[Lobbyist]

How do we know it was not placed there before the shooting?

[The Red Zone]

We would then expect to have cards found in non-shooting areas too (non-shooting because passersby made impossible the sniper's invisibility).

[LibWhacker]

Can't be sure, but if it was left there before the shooting by some third party trying to cash in on the situation, then he got unbelievably lucky. Either that or he (and perhaps a whole bunch of other creeps, too) have been scattering ransom demands all over the area, hoping they'd get lucky. But in that case, I'd expect some of those notes to have been found by now.

[demosthenes the elder]

"How do we know it was not placed there before the shooting? "
It may have been. However, that means it must have been placed by someone with foreknowledge of teh shooting, which means the sniper or an accomplice.