Isn't it obvious that the main purpose of "Homeland Security" is to reassure us sheeple that terrorism is never the cause of anything?
We are a country at war, they admit there are currently Al Quaeda cells operating in the U.S., so what are they doing, watching American Idol and going to baseball games? Oh I'm sorry that's what we're doing.
Homeland Security expects a terrorist group to call them immediately after the attack, to claim responsibility and then to actually prove that responsibility beyond a reasonable doubt. No idea what happened here, but we can at least be fairly certain that we are not under attack from Martians.