Seems to me a sniper or snipers wouldn't even need to be in view of the crowds or even very close to the stadium to cause havoc, given suitably calculated ballistics.
That's how I wrote it. The fictional sniper used an SKS to loft his bullets over the near side of the stadium and into the opposite upper deck from 1000 yards away, beyond the immediate police perimeter. In this fictional case, the sniper was not suicidal. A suicide bomber or bombers inside of the stadium would do just as well at triggering a panic stampede with mass casualties.