Where does the "heavily guarded airspace" bit come from? I get that the whitehouse would have been guarded (if the plane made it there), but wasn't everyone really lax about it everywhere else, before this happened?

The sort of thing where stupidity happens - plane crashes into a building, and then Air Traffic sees a second plane off course and thinks "oh he must be confused by all the panic" instead of "he's planning to do the some thing" -- because they can't get their head around the idea that someone would do that on purpose. Until someone did.