Dozens of people are dead after a regional jet collided with an Army Black Hawk helicopter Wednesday night over Washington, D.C., officials said, the nation's first major commercial airline crash since 2009.

This data point is one of several key mysteries investigators are exploring as they seek to explain what caused the nation's worst air disaster in more than a decade, aviation experts said.

The helicopter involved in the fatal midair collision on Wednesday night may have flown off its approved path, The New York Times reports.