From what I have gathered from reading and a pilot friend is that the FAA didn’t contact the military until the aircraft actually overflew MSP. So then they made the notification, F-16’s of the Wisconsin ANG were ready to go but in that 15-20 min period of notification and the planes getting ready to go up they got in contact with the NW aircraft so they had the ANG stand down. So that means they left Denver center airspace and made no contact with MSP approach, MSP tower, anyone, but no one realized it or did anything until they actually overflew MSP. It’s the breakdown in the chain of communication.
I wonder if they really did have aircraft ready to go. With all the budget constraints I was be concerned there's really no ‘on alert’ units in the heartland. When you think of a situation requiring them to scramble fighters you have a movie plot picture in your mind of pilots just sitting around in a ready room somewhere shooting pool and watching TV, just waiting for the klaxon horn to sound. But in real life, it takes quite a few people to get a fighter off the ground, and it's even more daunting if everyone is home, asleep and must be called in. That could take at least an hour.