The victim was exiting the bus when he got into a dispute with the assailant, Smith said, adding that the two were apparently members of rival gangs.
The gunman opened fire, shooting the victim in the head, exited the bus and fled, Smith said.
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Edward Walker of Los Angeles was declared dead at the scene, according an investigator at the coroner's office.
Walker's family members arrived at the scene during the investigation, an On Scene Video camera crew reported, adding that one relative was so distraught he had a seizure and was taken to a hospital by an ambulance.
LAPD gang and homicide detectives were handling the investigation, according to a watch commander at the LAPD's 77th Street Station.