Brothers Leland and River Bates' little bodies suffered extremely high core temperatures before they shut down, preliminary autopsies revealed.
The Bradley County boys, 5 and 3, died of hyperthermia, a condition brought on when the body's core temperature surpasses 104 degrees, specialists say. Normal body temperature averages 98.6 degrees.
"The temperature we normally would say is life-threatening is about 107 to 108 degrees Fahrenheit [and above]," said Dr. Erin Reade, pediatric care specialist at Children's Hospital at Erlanger.
The boys' mother, Tasha Moses, first called for help on June 28. River died that day. Leland, who was flown to Children's Hospital at Erlanger, died the next day.
Moses first told officials the boys had drowned.
Within a few hours after the incident, however, authorities reported that water wasn't involved in the deaths.
Moses told officials she had left her boys unattended on a Slip 'n Slide outside her home for 45 minutes. The high that day was 101, according to National Weather Service records. After finding the boys unresponsive, she told officials she drove them to her father's home about a mile away because she did not have a phone at her home.
Officials with the Bradley County Sheriff's Office have not released additional information from the autopsies, citing the ongoing investigation. They also have declined to say whether they plan to press charges.
Reade said most child victims of hyperthermia have been left in confined areas, such as closed vehicles, in high temperatures.
"By far, the most common way we see are children forgotten by a caregiver in a car, playing in an unattended vehicle, those kind of things," Reade said.
In such conditions, a child's body temperature can rise to lethal levels within just 30 minutes.
Children do not sweat as much as adults, she said, so they are unable to quickly rid their bodies of heat. The younger the child, the more quickly internal temperatures can climb, she said.
Above 107 to 108 degrees, she said, "you get shut-down of the basic processes in the body that need a normal body temperature to function."
Reade said the quickest way to help people suffering from hyperthermia is to immediately splash cool water on them and apply cool compresses and ice packs.