As Travis points out, a TC Contender in .223 would be optimal for all the work that that been done so far by the perpetrators. Remove from backpack, shoot, toss back into pack, bicycle away. From a vehicle, even easier.
As harpseal has mentioned elsewhere, the manner in which the perps have eluded law enforcement so utterly and completely so far is the single most powerful indicator of terrorists. They have the equipment, the training, the skills, the planning, the field support and the mindset, as so far quite capably demonstrated.
As Squantos has pointed out, the casings found by LE so far could have readily be souveniered off of a rifle range and left for the law to find as a red herring. Very easy to do and a very effective countermeasure.
I have little doubt that the perps chose this particular AO in large part due to the heavily disarmed nature of the populace. If such events were to happen here in Florida, the terrorists, although likely prepared to die, would be severely shortening their operational lifetime if they chose this area for their crimes. Where they are now, it's like shooting ducks in a pond.
As for stumbling on the shooter as he/she's about to fire and engaging with a handgun. It's doable, if one was to be so incredibly lucky. Get as close as possible without being detected, seek cover, call on cellfone to notify police and give location, and see what happens after that. If you deign to take a shot or two at the perp, you know they will very likely engage you. And when the law shows up, be prepared to be treated like the bad guy. If you can nail the goblin and disappear without notifying the authorities, engagement would be more of an option. You would still be putting yourself at extreme personal risk from both sides of the equation by doing so.
I must add that it's pretty apparent that the powers-that-be don't want to mention the terrorism aspect of this because they simply don't want to cause a panic. The herd is obviously already pretty spooked. The authorities have worked hard and long over many decades to breed a docile, defenseless and obedient population of sheeple, and they don't want these events to spoil it now. Reminds me of that curse about "be care what you wish for"...
Stay safe, everyone.
