"During the news conferences, they've reiterated they are not withholding info and have had no contact at all from anyone. Surely that rules out a terrorist group. Talk of using the plane etc seem so ott to me. There are much easier ways of doing things."
Maybe. But if they had further plans, or their plan went wrong, they wouldn't want to let the authorities know exactly what was/is planned.
Hijack situations used to be like that - hold hostages, make demands. 9/11 changed that.