I don't remember who was asking why the transponder would be shut off manually, but according to the video i just watched it's common practice to shut it off while on the ground but it's almost never shut off while in the air.
http://online.wsj.com/news/articles/SB10001424052702304185104579437573396580350?mod=trending_now_1
My guess as to what happened is it was hijacked and passengers or staff on board were somehow able to crash it into the ocean, hoping to spare the lives of people at the intended target, like what was done on 9/11.
http://online.wsj.com/news/articles/SB10001424052702304185104579437573396580350?mod=trending_now_1
My guess as to what happened is it was hijacked and passengers or staff on board were somehow able to crash it into the ocean, hoping to spare the lives of people at the intended target, like what was done on 9/11.