conducting a so-called vertical patrol

The officer, Peter Liang, and his partner were conducting a so-called vertical patrol on Nov. 20, 2014, inside the Louis H. Pink Houses in the East New York neighborhood. At one point, Officer Liang opened a door into an unlighted stairwell and his gun went off. The bullet glanced off a wall and hit Akai Gurley, 28, who was walking down the stairs with his girlfriend, and pierced his heart.