Two things concern me here. Could be some of those "other than Mexicans" that have been crossing the border down this way have got themselves organized; maybe in conjunction with the local drug dealers who could provide intelligence for money. The other is homegrown Tim Veigh types, either alone (2 or 3 persons) or as part of a homegrown milita group which may be in the area.
Tim Veigh = Tim McVeigh