A plane being flown into a building on that date, that city, at that time was known? Or was there a threat known, but no specifics to go on? There is a big difference. I believe the 2nd possibility is more likely.
"Okay, so the first plane took us by surprise. But 15 minutes later, we get surprised again......I don't follow that."
If you remember, the way it was reported live, was that the first crash was a terrible accident, not an attack. So with that being the mindset, it doesnt surprise me that they got away with the 2nd crash.