The crash happened around 1:15 p.m. Monday at the intersection of Olympic and 43rd avenues when a truck went through the intersection and struck a mid-size sport utility vehicle, police Lt. Mike Brunicardi said.
Both vehicles then traveled into the Mollie Stone's grocery store parking lot and struck seven other vehicles and a pedestrian, Brunicardi said.
The 9-year-old boy, who was inside one of the cars that was hit, was killed, and three women, ages 30, 52 and 53, were hurt.
The name of the boy who died has not yet been released.
One of the injured women was taken to the hospital via helicopter and the other two were transported in ground ambulances.
The 30-year-old woman and 53-year-old woman were admitted to an intensive care unit and the 52-year-old woman was kept at the hospital for observation, Brunicardi said.
Brunicardi today would not confirm witness reports that one of the injured women is the mother of the boy killed in the crash.
A man who answered the phone this morning at Hayward-based Century Roof Tile, a roofing company, said the truck involved in the crash was one of the business' service trucks and was driven by an employee. He said the business is waiting for the Police Department to complete its report before commenting on the situation.
Brunicardi said a California Highway Patrol investigation team is working to determine whether the truck crashed into the SUV due to a mechanical failure.
"We don't know whether this was a mechanical issue or not," Brunicardi said.
A worker at a nearby paint store said he had heard several people say the truck's brakes had failed.
Brunicardi said the driver of the truck was the vehicle's sole occupant and has been cooperative with investigators.
He has not been cited or arrested, but police and the CHP will forward their completed investigations to the San Mateo County District Attorney's Office to determine whether charges will be filed, Brunicardi said.
"We are not eliminating anything at this point," Brunicardi said. "We are going to take the time necessary to make this an accurate and true reflection of the incident."
He saddened when he spoke of the crash, mentioning that he himself is a father.
"This is just a horrific, life-altering incident," Brunicardi said.