Gecko45, SpecialForces:
Perhaps your security measures regarding this are classified - and this is may be why you have not mentioned it - but I think your biggest vulnerability now is lack of communications integration with the local / federal authorities and inadequate standoff range capabilities. You may also want to consider a special hardened sucurity room should the worst occur.
Consider this: A terrorist nut with an explosive laden UPS truck is careening towards your mall. When will you know? Have you trained with the local PD to get advanced notice of suspicious vehicles heading your way? Do you position your own agents to overwatch the high speed avenues of approach to your retail establishment? Even slow moving, otherwise peaceful looking vehicles may be a threat. How do you currently search incoming traffic - and is this far enough away from the actual mall that an explosion at you checkpoint will not effect shoppers? If not, you could be very well at risk of a serious Kobar Towers / Murrah Building type incident.
How about if a suspicious aircraft veers off course and heads in your direction? Have you tested / trained on communications response times with the folks at the local air traffic control tower to be sure you will have early enough notification?
Then there is the issue of standoff range. Should you receive prompt notification of such threats, what do you have to neutralize them? I strongly recommend an ample stock of anti-tank rocket launchers (LAWs will do for most threats, but one or two Javelins would be important in case the enemy were to hijack a real tank). In addition, air defense will require some serious thought. Surely you must accept that your high speed sniper rifles - while very efficient at taking out the gray haired old ladies who try to steal napkins from the food court - would be of little use against a threat like that which attacked the world trade center towers. Should Stingers be beyond your department budget, surely some old Redeyes will defend against such non high performance aircraft as jumbo jets and crop dusters. You may just have to gamble on the terrorists not getting hold of F16s or such.
Lastly, should any of these things happen and you are unable to stop the attack, it will be critical that key members of your security force survive the incident. You must not allow anarchy and chaos to reign on what had once been a carefree (at least for the Civilians), safe shopping establishment. This is why I recommend that you have a heavily reinforced command post that would serve as a sort of bomb shelter should the unthinkable occur. That way, much of your force would be more likely to survive the first assualt and be around to continue to secure what is left of the area.
One last note, if you do implement such measures, it will be very important not to let specifice details get into the wrong hands. You may want to look into issueing cyanide pills to each of your agents. Even the best of us may quastion our ability to withstand torture. Men and women like those you work with, though, would surely appreciate the chance to not have to risk any possibility of revealing security details. They could take the pill and remove themselves as a risk.
Ever vigilant,
Adam