Imagine a well coordinated attack by an entire terrorist cell. After getting through the perimeter they would lite up the weapons storage facility. Now all that stood between the terrorist cell and a few hundred highly trained and defenseless US military personal would be a few dozen MP's and some civilian police. And the civilian police wont stand a chance against assault rifles. I guess now we know why there has been plots uncovered in Australia and the US where terrorists were going to attack domestic military installations. Always sounded like a stupid plan to me, but perhaps the terrorist found a weak spot.
Unless they have M4 Carbines in their trunks. Many civilian police have those, or similar, in their trunks.