The shooting happened around 1 a.m. Saturday near Ocean and Colorado Avenues. It was unclear how many suspects were being sought in the shooting.
Police earlier said one arrest had been made, but have since said that the arrest was not related to the shooting.
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"One of the buses had a group of people that got off the bus, got into an altercation of some sort with another group that was on another bus that was directly parked behind it, at which point shots were fired," explained Lt. Saul Rodriguez, of Santa Monica police.
Officers determined that after an argument between a person from the first bus and people from a second bus parked on Ocean Avenue, suspects from the second bus fired multiple shots at the first bus, striking three victims on board and another on Ocean Avenue.
The driver of the bus that was shot at drove to a nearby police station with the wounded passengers, who were celebrating a birthday at the time of the shooting.
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A female victim, who has not been identified, died of her injuries, authorities said.
Police said they do not believe the two groups knew each other.
Anyone with information on the suspects or the shooting was asked to contact Detective Leone at (310) 458-8949, Detective Cooper at (310) 458-8949 or the police department at (310) 458-8478.