Since your signup date is Aug 27, 2015, I have two names for you, Jamie Gorelick and Bill Clinton.
The Gorelick memo of 1995 erected a "wall" between counter-intelligence and law enforcement, which impeded investigation of al Qaeda in the run-up to the 9/11 attacks. Jamie Gorelick, then working as Deputy Attorney General under Janet Reno was author of the memo.
If Bush II was a serious man before 9/11 (He WAS after) then he would have recognized the danger in delinking agencies involved in security. Sorry, but this is NOT complicated. When the nation’s worst terrorism disaster happens on YOUR watch you own it. If that burden seems unfair, DON’T RUN FOR THE OFFICE.