In the movie Minority Report, there is a lot of build up to the nation wide release of PreCrime. But in the movie the precogs only have a range that entends to the greater D.C. area. And the seemingly retired doctor which accidentally created them didn't seem like the type to make any more.
The precogs clearly have some range, because at the end of the movie they are placed on a distant island far away from people. (I know there is some speculation that the ending is a dream).
If we are to believe that the precogs see all murders everywhere, it would be a never ending stream of murders. How would they isolate murders in D.C. compared to everywhere else?