There are many agencies and organizations that should be involved in creating such a plan.
Among the obvious ones are first responders. Police and fire and emergency medical personnel must surely be involved in the creation of such a plan because they would be the ones most directly involved when an incident first happens.
Then, there would need to be involvement on the part of any major industries in the area. Any firms that have things like chemical tanks that could be blown up, causing danger to the community would need to be involved in a plan. Any dams in the area that could be destroyed, flooding the area would need to be included.
Finally, there should be some involvement by community groups that could be counted on to help deal with the aftermath of an incident. This would be groups like churches that would have space to house evacuees or fraternal organizations that would have networks in place to secure things like supplies for people in shelters.
All of these would need to be involved in the creation of a good security plan.