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.

A preliminary report into last month’s fatal Air India crash is expected as early as Friday, Reuters reported on Wednesday, citing three people familiar with the investigation. The focus of the probe has narrowed to the positioning of the aircraft’s engine fuel control switches.

The warning affected a narrow, but densely populated area that included downtown Chicago. Flash flooding was seen from the downtown area through the West Side along the Eisenhower Expressway to Oak Park, as the slow-moving storm hovered stubbornly over the area.