A month after the worst aviation accident in India in decades, authorities are poised to release a preliminary report into the catastrophe, offering the first official clues to what caused the aircraft to plummet back to the ground just 30 seconds after taking off.

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.

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