I’ve already shared Sam Savage’s take on the failure to keep the underpants bomber from getting on that plane to Detroit: that it’s just really really really hard to identify a vanishingly small segment of the population and keep them off airplanes without mistakenly preventing scads of harmless people from boarding. But there are other angles, one of which President Obama addressed Tuesday (and will presumably address again in more depth this afternoon)(update: he did):

[E]lements of our intelligence community knew that Umar Farouk Abdulmutallab had traveled to Yemen and joined up with extremists there.  It now turns out that our intelligence community knew of other red flags — that al Qaeda in the Arabian Peninsula sought to strike not only American targets in Yemen, but the United States itself.  And we had information that this group was working with an individual who was known — who we now know was in fact the individual involved in the Christmas attack.