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.

The investigation into last month’s Air India crash is focusing on the actions of the jet’s pilots and doesn’t so far point to a problem with the Boeing 787 Dreamliner, according to people familiar with U.