Atlanta (CNN) -- Open a car door on a summer day, and a sauna blast will quickly remind you just how seethingly, sticky hot it can get inside in little time. It's suffocating.
For 22-month-old Cooper Harris, strapped all day into a child's seat in his father's SUV, as the sun baked it, it was fatal.
Investigators in Georgia wanted to know how high the temperature climbed in that back seat, so this week they recreated that sauna heat in Justin Ross Harris' silver Hyundai Tucson.
They drove it to the spot where it sat in the heat for seven hours on June 18, the day Cooper died.
They parked in the same space that Harris did, CNN affiliate WAGA reported, and measured the temperature at times of day that are key to the father's felony murder case:
-- At 9:30 a.m., when police say Harris pulled into the parking lot at Home Depot's corporate offices, where he worked. He normally would have taken Cooper to daycare then but left him in the car.
-- At 12:42 p.m., when the 33-year-old father placed light bulbs inside the car.
-- And at 4:16 p.m., when investigators say Harris drove off.
Up to 140 degrees
They have not released the data yet, but CNN weather experts believe temperatures could have climbed to nearly 140 degrees.
A government traffic agency has corroborated the possibility.
"Cars parked in direct sunlight can reach internal temperatures up to 131 degrees F -- 172 degrees F when outside temperatures are 80 degrees F -- 100 degrees F," the National Highway Traffic and Safety Authority said.
"Even outside temperatures in the60'ss can cause a car temperature to rise well above 110° F."
On the day Cooper died, the high temperature reached 92 degrees. Investigators used outside thermometers on Tuesday to monitor outdoor temperature rises.
Dozens of children die in hot cars every year, the NHTSA said.
People are in danger of dying of heatstroke when their body temperatures climb above 104 degrees and stay there for prolonged periods, according to the Mayo Clinic.
Heat attacks the brain, heart, kidneys and muscles, the Mayo Clinic said.
Victims can experience nausea and faintness, before organ damage sets in, eventually leading to death.
The elderly and small children are particularly susceptible to heatstroke.