There are hundreds of “soft target” — unguarded, no personal firearms allowed on premises, many civilians etc — such as shopping malls, theaters, nursing homes, office buildings, all school campuses, bars, on and on and on, which would be “easy pickings” for mass casualty-oriented terrorists. Minimum effort, maximum high casualty opportunities.
This is true, but the most emotionally compelling targets are day care centers and elementary schools. These should wise up and be prepared.