Most people who provide useful information about those who pose threats to our nation do it under the threat of prosecution (or to win leniency for what would otherwise be mega-sentences). Obviously, upstanding members of the community tend not to know much of anything about the inner-workings of terrorist organizations — it's the bad guys you must induce to cooperate. For all the admitted problems of marrying up competing skill-sets, the reason combining the law enforcement and intelligence functions makes sense is because you can seamlessly leverage prosecution to obtain intelligence. Wouldn't we be losing a lot by dividing those functions? An MI-5 is going to want to horde its sources for pure intelligence purposes rather than turn them over to the FBI and DOJ for prosecution; but if you don't threaten them with prosecution, you very likely will not get the most deeply held intelligence secrets.
Britain’s 7/7 bombings demonstrate that having a dedicated domestic intelligence agency is far from fool-proof. (Judge Posner, to his great credit, has been superb in explaining why intelligence is such a difficult challenge, and why we may never have a very high batting average when it comes to preventing attacks.) While his piece today is typically thought-provoking, I don’t think it makes the case that an American MI-5 is either possible or necessarily desirable.
-- Andy McCarthy