Exactly..dead people can’t turn off transponders..and you have to be extremely knowledgeable about planes to do so, so the pilots(Or someone else, like a hijacker) had to have turned them off..remember, all of the 9/11 hijackers, the second they took control of those planes what is the first thing they did, turn OFF the transponders
Just someone who was trained to do it. Don't even have to tell them why, just do it.
John Nance said that there were two different tracking/ radar systems that were turned off at different times. He also said that whoever did it had to have knowledge of a 777.