It seems to me that your plan was sort of tried tonight in Virginia.
For a much more rigorous enactment of your plan, the President would need to declare martial law.
A number of people on this thread are not going to cooperate with declaring martial law, so it may not be politically viable to do the logical thing you suggest. They are more frightened of their government than they are of terrorists.
Someone stated she will cooperate, IF the government first proves to us that it definitely foreign terrorists.
The problem is that the proof for or against that possibility can probably only come after the murderer(s) is(are) caught.