My guess is airline employees working with terrorists are responsible for both.
The first one broke up in mid-air, the second one sent out the hijacking signal and then went down and crashed into the ground. Sounds like the second one did not break up in mid-air.
Don't know how they did it but they did. Many don't realize, that if someone is intent on bringing down an airliner, it can be done. Just glad I don't have to fly anymore. Even without the wacko terrorist, it was a pain in the butt.