I am looking at this from a variety of view points.
The LEO were counting on the fence to keep people out and as perimeter security. Were they having random patrols check the fence area?
Would this be considered a gun free zone? Could the argument be made as a gun free zone, no CCW, effectively rendered all the attendees defenseless? Could a CCW carrier have engaged and neutralized the shooter before the shooter could have killed/injured more after the first shot?