Therefore, if they are not local, they will need to be in some form of accomodation. So hotels, motels, Bed and Breakfast, cheap room only accomodations need to be checked.
Otherwise they could be deliverymen, who continually go around this entire area, so if stopped they would have a perfect excuse for being there like; showing the policeman the meat in the back, having delivery notes of exactly where they are going too etc.
If they do live in the area and it is two people, then someone must know them, no two people are that crazy and stick together that much that somone would not know them.
My last possible is that they just got out of prison and want to take revenge on anyone in that area for something that happened to them or their family or maybe the reason why they were in Prison. This way the two would have spent a long time together in jail or maybe one is from jail and the other is the younger brother who is doing the driving.
Other wise next time their is a shooting, have the national guard ready and lock it down, or at least 20 square miles, nobody gets in nobody gets out and if necessary a curfew, for a couple of days.
Gotta Catch them and Gotta do it Now.