Story highlights Police: Child appears to have locked self inside hot car

Experts warn parents to keep doors locked in parked cars to protect kids

(CNN) A 3-year-old-boy who locked himself in a hot car and died in Texas is a tragedy that happens more often than people realize, a safety expert told CNN Friday.

On Thursday, the boy in Houston entered the car parked near a house to retrieve his toy from the back seat, said Kese Smith, spokesperson of the Houston Police Department. He entered through the unlocked front door and climbed into the back seat.

Once he got into the back seat, he couldn't open the back door because the child protective locks were on, Smith said.

Since 1998, more than 660 children across the United States have died from heatstroke when unattended in a vehicle, and 29% of those kids were playing in an unattended vehicle or gained access on their own, said Amy Artuso, a program safety manager for the National Safety Council.