The real problem is flagging 0.2% of the population is useless when 0.00001% of the population is terrorists.  If everything works perfectly your people end up investigating 1,000 innocent people in a row which trains them that everything is a false positive.<p>The 9&#x2F;11 hijackers and the Boston marathon bombers where both flagged yet the attacks still happened in large part because there was simply far to much noise from other people and not enough signal.