Bus line 40 was traveling northbound on Crenshaw Boulevard when a passenger pulled out a gun and shot another passenger just after 9 p.m.
The shooting happened as the bus came to a stop at Crenshaw Boulevard and West 60th Street in Hyde Park.
Only a single shot was fired, but it killed the victim, identified as 17-year-old Edward Walker III.
The gunman fled the scene, along with some of the passengers who were on the bus.
Investigators combed the bus, looking for evidence that might help find the killer. Police believe the suspect may still be in possession of the gun.
Detectives said on Tuesday morning that they really need to talk to everyone who was on that bus. The gunman is on the loose, and he remains a danger to the community.
The suspect is described as a male black, 18 to 22 years of age. He was wearing a dark or black jumpsuit at the time of the shooting, and he was also wearing a dark knit cap.
Anyone with information is urged to call (877) LAPD-24-7.
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