They were out of contact for an hour and a half, not just during the time they were beyond their destination.
I suspect one reason there wasn’t more alarm on the ground was that their transponder was still squawking and their course did not change unexpectedly. In most hijack situation they disable the transponder right away.
But I am surprised they didn’t send a military plane up to get a look at the aircraft and see if there was anything obviously amiss. I would have expected them to be sending someone out to take a look pretty quickly after contact was lost, and especially when they deviated from their IFR clearance.
It will all come out sooner or later. I can’t imagine ANY scenario that will turn out OK for these pilots. Expect maybe they were part of a TSA test of the system to see what ATC would do under the circumstances. And I don’t see that as very likely.
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.