Actually, I'd advocate about 200 local hunters with scoped rifles aimed at the terrorists from every possible angle, set to go on signal. With two or three shooters per target, things should be settled pretty quickly. A SWAT style entry team or two could mop up. The locals know their rifles, and ranges could be figured to the foot.
If an attack were to take place in Illinois, the police would be rounding up all law-abiding gun owners that are registered with the Illinois State Police's FOID (Firearm Owner's Identification) database.
An attack would be a dream come true for Illinois socialists.