My guess is that it’s more a trial of how the USAian public will react to such a move. El Paso was probably chosen due to the high number of potential distractions (aliens, whether extraterrestrial or human), potential invasion of Mexico, limited airstrikes in Juarez, all the other stuff discussed in this thread, etc.
The real motivation is probably to find out how much they can get away with in the event they decide to do something like this again for some other nefarious purpose.