Something bad is going to happen tomorrow
You've been warned. Now, what are you going to do about it?
What, I didn't provide enough information? Hell, I gave you a specific date. I even said "something bad" is going to happen. Isn't that enough?
A non-specific mention about crashing a plane into the Pentagon or the White House isn't exactly enough to do much about. There are literally thousands of airline flights in the US every day between dozens of major airports. Without something more definitive like a date or an airport, how much can you heighten security and accomplish anything? Furthermore, at the time of the 9-11 hijackings, those boxcutters they carried on the planes were completely legal. Even if they'd been detected by security, they would've been allowed through.