I thought we had tight security. Just think what a plane could carry into downtown. Think what 4 guys could carry to a rooftop next to the center. 400 lbs of C4? a couple of '50's with plenty ammo? few RPG's maybe? This is rediculous if they aren't cops. If they are, couldn't they take the stairs?
Just dump some WMD into the HVAC units. Everyone would be dead a few minutes later.