Johnny Rhoades of Shelby Township told the Detroit Free Press he found his grandson, James Nelson, 2, when his daughter's roommate came to him for help Tuesday evening.
"She's pounding on the door. I go down there, and he's dead," a sobbing Rhoades told the newspaper. "He's in the van, strapped in his ... car seat, sopping wet, blue. My girlfriend tried CPR, but he was dead. The poor baby."
Investigators questioned the boy's mother, Adrianna Rhoades, for hours, her family said. They were trying to determine who left the boy, strapped in a carseat, inside the van.
While temperatures were not high in the area Tuesday, experts say a closed car or van can become far hotter than the outdoors if left exposed to the sun. Young children are especially vulnerable to heat stroke because their bodies heat up much faster than adults.
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