An official report on the world’s deadliest aviation accident in a decade, seen by CNN, has answered one key question – but raised others.

Last month, 260 people were killed when Air India’s Flight AI171, a Boeing 787-8, crashed on its way to London Gatwick due to failed engine fuel control switches. The flight crashed on June 12 shortly after taking off, killing 241 out of the 242 people on board, along with 19 people on the ground.