There is speculation that the crew accidently turned off the transponder.
This is not the cause of the accident. Aircraft experience transponder malfunctions every day. Some of these malfunctions are simple "finger problems".
Thanks for directly answering. I deleted my previous post. One cannot so easily answer to provocations of some junk minded people (not your case).
But anyway, shouldn't the crew monitor the a/c instruments, moreover when they are related to navigational information ?
What I am just trying to say is that this accident was caused by a series of mistakes from both the legacy crew and the Brazilian ATC. However I disagree with ATC4US when he says that the legacy crew should be responsible for not having changed its FL.