As the world waits for answers on the fatal Air India Boeing 787 crash, accident investigators have narrowed their focus on the engine fuel control switches.

The London-bound flight crashed shortly after takeoff, descending rapidly from 650 feet and killing 241 of the 242 people on board, along with casualties on the ground.

A preliminary report into the deadly crash of an Air India jetliner in June is expected to be released by Friday, three sources with knowledge of the matter said, with one adding the probe had narrowed its focus to the movement of the plane's fuel control switches.