The new documentary "Down a Dark Stairwell" re-examines a tragic 2014 incident when Chinese-American police officer Peter Liang shot and killed an innocent, unarmed Black man named Akai Gurley while both men were in a dark stairwell at the Pink Houses housing project in Brooklyn, N.Y. Liang had been conducting a routine patrol of the property when he discharged his firearm, but the bullet ricocheted off a wall and fatally struck Gurley. Thus, two marginalized ethnic communities found themselves thrust into the uneven American criminal justice system together.