The New York City police officer who fatally shot Akai Gurley took the stand to give his account of the deadly shooting.

Officer Peter Liang began testifying in his own defense on Monday. He’s charged with manslaughter in the death of Gurley, who died after being shot in a public housing stairwell in 2014.

In his testimony, Liang described the “vertical patrols” executed by police in stairwells. He noted that the stairways can be sites of crime and danger.

Liang, along with his partner, were patrolling a dark stairway in a Brooklyn housing complex when Gurley was killed. A bullet, fired from Liang’s gun, ricocheted and hit Gurley a flight below.

Liang’s lawyers contend that the shooting was an accident, not a crime. Prosecutors say Liang handled his weapon recklessly and did nearly nothing to help Gurley after he shot him.

Read more at ABC News.