The Wall Street Journal, July 16, 2009 by John Yoo
http://online.wsj.com/article/SB124770304290648701.html#printMode
Unlike, say, Soviet spies working under diplomatic cover, terrorists are hard to identify. Yet they are vastly more dangerous. Monitoring their likely communications channels is the best way to track and stop them. Building evidence to prove past crimes, as in the civilian criminal system, is entirely beside the point. The best way to find an al Qaeda operative is to look at all email, text and phone traffic between Afghanistan and Pakistan and the U.S. This might involve the filtering of innocent traffic, just as roadblocks and airport screenings do.