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 just a short 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 sun for seven hours on June 18, the day Cooper died.

They have not released the data yet, but CNN weather experts believe temperatures could have climbed to nearly 140 degrees inside the car.

The National Highway Traffic Safety Authority has corroborated the possibility.

"Even outside temperatures in the 60s can cause a car temperature to rise well above 110° F," the agency said.

The test came in the same week that the Cobb County medical examiner's office said toxicology tests on the boy revealed nothing abnormal, meaning he apparently was not drugged or medicated.

That report and the autopsy report -- which found the child's cause of death "consistent with hyperthermia" and that investigative information "suggests the manner of death is homicide" -- will not be released to the public until the investigation is complete, the office said Thursday.

Cooper's father, Justin Ross Harris, is charged with murder and child cruelty. He has pleaded not guilty.

Harris fired by employer

Harris, who worked for two years at the The Home Depot's corporate offices in the Atlanta suburb of Smyrna, has been terminated, a spokeswoman for the company said Thursday.

Harris, a web designer, had been on unpaid leave since charges were filed last month.

Catherine Woodling gave no details of the termination.

A Home Depot charity fund paid for Cooper's funeral.

Measuring temperatures at key times

During this week's car test, investigators parked in the same space that Harris did, WAGA reported, and measured the temperature at times of day that are key to the father's murder case:

-- At 9:30 a.m., when police say Harris pulled into the parking lot at The Home Depot's corporate offices. 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 he had purchased inside the car.

-- And at 4:16 p.m., when investigators say Harris drove off.

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.

Uncomfortable details

Initially, police said the death of the toddler was the result of tragic absent-mindedness.

They said the dad had apparently forgotten the boy was in the back seat of his Hyundai Tucson and apparently didn't remember until he was done with his workday, drove a couple of miles and pulled into a shopping center parking lot.

But suspicions grew as police investigated, and Harris was charged.