Gang enforcement officers Allan Corrales and George Diego heard a loud noise while driving in their patrol car and turned around to see Washington. The officers said he was looking around suspiciously and manipulating something in his waist area.
When the officers tried to stop Washington to investigate, he quickly approached them and appeared to pull something from his waistband, Assistant Chief Earl Paysinger told reporters at a news conference Saturday afternoon.
Each officer fired once and Washington was hit in the head, police said.
The officers fired "with the belief that he was arming himself, and in defense of their lives," Paysinger said.
Corrales and Diego called paramedics, who pronounced Washington dead at the scene.
Washington's family was told of his death later Saturday morning.
Family members said at the news conference that Washington was autistic and had never shown any signs of violence or had any trouble with police.
"They assured us that they are investigating, they also assured us that they care, so all we want is answers," aunt Vickie Thompson said.
The LAPD's Force Investigation Division and the district attorney's office are investigating the shooting.
Corrales has been with the LAPD for nearly seven years, Diego for nearly eight, police said.