His parents realized the child was left inside the car, and went back outside to get him — "where they found him unresponsive," the San Antonio Express-News reported.
"This really was just a case of parents who just got distracted," San Antonio Police Lt. Jesse Salame said. "They discovered him unresponsive in the vehicle, pulled him out and began life-saving measures, but were unsuccessful."
Salame described the situation as a "horrific tragedy," and said both parents thought the other had taken the child out of the car.
Temperatures in San Antonio on Saturday reached a high of 93 degrees, according to the news outlet.
According to Kids and Cars, a nonprofit organization that tracks such deaths, the boy in San Antonio was the 43rd child to die in a hot car this year — the sixth in Texas.
Dr. Rosina McAlpine, a parent expert and CEO of Win Win Parenting, previously told Fox News that oftentimes when parents forget their children inside hot vehicles, it's because they're often in a rush to get to work, an appointment, or another obligation and "forget they haven't dropped the child off at day care or school and rush off to the meeting or work distracted, leaving their child behind."
"Later they remember in shock but often it is too late," she said, noting some parents alternate dropping their child off at day care, and might "forget it was their turn." McAlpine said some caregivers "knowingly" leave children inside cars, as the kids "are sleeping and don't want to disturb them in the hope they'll get back before they awake."
Those who leave their children in the car also may not understand the danger they're in, or consider how fast cars can heat up. The interior temperature of a car can quickly skyrocket, with 80 percent of the total temperature rise happening in the first 30 minutes a child is inside a car — where the temperature inside can exceed those outside by up to 50 degrees.