Or even if they crashed, say a half dozen crop dusters loaded with kerosene or gas into 6 transformer stations out in the SW. Lightly defended targets accessible from air, and you don't have to hit too many at once to bring down 3/4s to 1/2 of the nationwide grid.
You seem to see the key angle of this campaign - the high body counts are frosting on the cake, which is an assault on the US economic infrastructure, which is the main things these nuts really hate about us. Transportation systems, mail systems, power grids, computer networks - these are the likely targets of the evil ones.
Maybe it's connected to the 20 "Suitcases" these guys are suspected to have in their possession -- by way of the Russian Mafia.