There is a small button that pilot/copilot can push easily when a plane is hijacked, that sends distress call and reports that the plane is being hijacked. It's discrete and can be done very easily. Unless the Hijackers took control of cockpit without the pilots being able to push the butttons, it's very hard for that signal not to be initiated. No such signal was initiated so I highly doubt this was the case.

Either pilot error (stalling) or plane just blew up to pieces before the pilots realized, are my guess.