Four pedestrians were running through a crosswalk at Paramount Boulevard and Slauson Avenue about 3 p.m. when one of them was struck by a box truck that did not stop, the Los Angeles County Sheriff's Department said in a news release.
The driver continued eastbound on Slauson Avenue as witnesses noted the vehicle's license plate number, investigators said.
The 15-year-old victim was transported to a hospital in critical condition, the sheriff's statement said, and later died at the medical center. His identity was not disclosed.
Shortly after the collision, authorities said, Whittier police officers assisted sheriff's deputies in finding the vehicle believed to have been involved in the incident.
A person of interest was detained, officials said. The individual was not publicly identified.
Anyone with information about the collision is asked to call Traffic Detective Marquez at the sheriff's Pico Rivera station at (562)949-2421. Anonymous tips may be submitted to Crime Stoppers at (800) 222-TIPS (8477).