Officers fired on an assault suspect and a bullet went through the wall and struck the girl, authorities said.
Police also fatally shot the suspect Thursday morning at a Burlington store in the North Hollywood area of the San Fernando Valley, police said.
Witnesses told KCBS-TV that a man began acting erratically, threatening to throw items from the upper floor, and attacked a woman with a bicycle lock shortly before noon as the store was crowded with holiday shoppers.
Officers answered a report of an assault and others of shots being fired, police said. Investigators have not found a gun at the scene.
The suspect was shot and died at the store but one of the bullets went through drywall behind the man and killed the girl, who was in a changing room with her mother, police said.
Officers found the teenager dead after seeing a hole in “a solid wall that you can't see behind,” LAPD Assistant Chief Dominic Choi said.
Investigators didn't immediately know whether she was in the dressing room before the violence began or ran in there to hide, he said.
“This chaotic incident resulting in the death of an innocent child is tragic and devastating for everyone involved,” Police Chief Michel Moore said in a statement late Thursday night. “I am profoundly sorry for the loss of this young girl’s life and I know there are no words that can relieve the unimaginable pain for the family.”
Moore promised a “thorough, complete and transparent investigation” into the shooting and said a critical incident video that will include 911 calls, body camera and other video will be released by Monday.
The names of the girl and the suspect weren't immediately released, and the woman who was attacked is not being identified.
Investigators were trying to determine whether the assault was random or targeted. Choi said they don't believe the teenager was related to the person who was attacked.
Police found a heavy metal cable lock near the suspect, Choi said.
The California Department of Justice was investigating the shooting, Attorney General Rob Bonta said.
News reports showed a woman with a bloodied face, who appeared to be the assault victim, being placed in an ambulance.
The victim was taken to the hospital with moderate to serious injuries, Fire Department spokesperson Nicholas Prange said. Choi said she had wounds to her head, arms and face.
The shooting recalled a July 21, 2018, confrontation in which LAPD officers accidentally shot and killed a woman at a Trader Joe’s market. Officers got into a gunfight with a man who authorities say shot his grandmother and girlfriend before leading police on a chase that ended when he crashed his car outside the market.
A police bullet killed Melyda Corado, 27, the assistant store manager, as she ran toward the store’s entrance after hearing the car crash.
The suspect, Gene Evin Atkins, took employees and shoppers hostage for three hours before surrendering, authorities said.
Atkins has pleaded not guilty to the killing.
Prosecutors found two police officers acted lawfully when they returned Atkins’ gunfire.
Additional reporting by The Associated Press.