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.
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.
Investigators also have unearthed uncomfortable details in Harris' online activities. He has performed Internet searches on child death in hot cars, they said.
While Cooper was left in the car, Harris was allegedly chatting via an online contact service with women. Police say that Harris, who is married, has, in the past, sent sexually explicit messages and photos on the service, including to an underage girl.
Leanna Harris, Justin's wife, has not been named a suspect in the case. Officer Michael Bowman, a Cobb County police spokesman, said Monday, "Leanna Harris has been interviewed. Detectives continue to work on the case."
Police have alleged she behaved strangely in the days before and moments after the death of her 22-month-old boy.
Thursday, defense attorney Lawrence Zimmerman confirmed to CNN he had been retained by Leanna Harris.