They had a total of 7:13 flight time, the flight was planned for 5:55. Backup fuel leaves them with about 7-8hr total. Either both pilots were taken hostage and flew it, the hijackers flew it and knew what to do, or the pilots were on a suicide mission.
All three are very plausible. This plane is in the ocean and won't be found for a long time, if ever.
at this point it's honestly best to just stop, the black boxes aren't going to be putting off signal forever, and the places it could be is enormous. A major loss for the families but this will take very long to fully get a definitive answer on what happened.
It's unlikely that if it crashed not a single piece of wreckage would be found, or that the transponders would be turned off 14 minutes apart.
Imagine this thing is loaded up with nerve gas or irradiated material and crashes into a major metro area? Fuck...
I hope the US is monitoring the coasts VERY closely. Even so, if the plane is flown low enough it can't be undetectable by radar.