There was no indication of mechanical failure or electrical glitch.
It wasn't until investigators listened to the cockpit voice recordings that they began to understand.
This airbus --one of the largest and most sophisticated aircraft ever built, a plane designed to be
an error-proof of automation--was at the bottom of the ocean not because of a defect in machinery,
but becouse of a failure of attention.

この部分の意味はどういう意味でしょうか？ Itは何をさしているのでしょうか？
It wasn't until investigators listened to the cockpit voice recordings　that they began to understand.