Carina Mancera, 26, and her 4-year-old daughter Jennabel Anaya were walking home in August with Jennabel's father Luis Anaya, when a gunman approached them at 9th Street and Locust Avenue and opened fire.
The bullets missed Luis Anaya, but hit Carina Mancera and Jennabel Anaya.
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Detectives released video of a person of interest in the case back in September.
Authorities announced on Wednesday that the man was named the suspect in the shooting.
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Investigators said the suspect was carrying a suitcase and boarded the Blue Line at 5th Street and Long Beach Boulevard, just five blocks from the crime scene, shortly after the shooting.
Passengers on the Blue Line told police the man talked about leaving California.
The motive behind the shooting remained a mystery to investigators.
A $40,000 reward has been offered for information leading to the arrest and conviction of the gunman.
MORE: $40,000 reward offered in fatal Long Beach shooting of mother, young daughter
Anyone with information that could aid detectives in identifying the suspect was urged to call the Long Beach Police Department's Homicide Detail at (562) 570-7244.