Firefighters and police responded around 3:40 p.m. to a medical emergency call at McCormick Street and Bluebell Avenue. Religious first responders also arrived.
Crime-scene tape blocked off an area of the neighborhood, and a white tent covered a part of a vehicle.
"An unresponsive child in a vehicle was discovered," LAPD Capt. Warner Castillo told reporters.
LAFD personnel pronounced the child dead at the scene.
At least two neighbors say a father had driven the neighborhood carpool in the morning and didn't realize a young girl never got out of the vehicle.
Police initially told ABC affiliate KABC that the child was a 4-year-old boy.
Attorney Lou Shapiro said the man involved is not the father of the child. Shapiro said he received a call from the man's wife.
"She's distraught. There's no words. She feels awful about this whole situation," Shapiro told reporters.
The abused child unit within the LAPD's Juvenile Division is investigating what Castillo said was a "possible homicide." LAPD said no one is currently in custody.
Police continue to interview witnesses as they work to establish a timeline and determine the circumstances that led to the child's death.
The death has shaken the devout Jewish neighborhood, where neighbors and families are deeply interconnected.
"These are families that all know each other in the community. There is only love for one another," Shapiro said.
It's unclear how long the child was inside the vehicle. Tuesday's temperatures in the San Fernando Valley reached into the upper 80s.