Autopsy results released a week after the death of the child ruled Cooper had died from hyperthermia and that the "investigative information suggests the manner of death is homicide."
Hyperthermia occurs when a person's body temperature rises above 98.6 degrees and remains there. Toxicology results from the autopsy are still pending.
Police were forced to defend the decision to charge Mr Harris after more than 11,000 people signed a petition calling for him not to face charges over the tragic death.
The petition, posted on the Change.Org. website states: "This is a horrible accident. The father loved his son immensely. These were very loving parents who are devastated. Justin already has to live with a punishment worse than death."
"Sending what's left of his family in to bankruptcy to defend him against these charges is only bringing more hardship to a family that will never recover from the loss of a child."
"There is nothing to indicate that the father intentionally left his child in the car, so a charge of murder is not appropriate."
The petition continued to gather signatures today despite unconfirmed reports alleging that Mr Harris had previously searched online for how long it would take for an animal to die in a hot car.
In a warrant filed against Mr Harris police claim that the suspect briefly returned to the SUV through the driver's side door to place an object in the vehicle at around lunchtime on the day of the child's death.
After leaving work around 4:15 p.m., Harris then pulled over at a nearby shopping centre and asked for assistance with his child.
Witness, Dale Hamilton, told the KPHO news channel that Mr Harris stopped his car across two lanes of traffic and attempted to revive him. Mr Hamilton also said Harris had to be restrained by police as he was so distraught.
"He pulled him out of the car seat, laid him on the ground and was trying to resuscitate him", Hamilton said.
"He was constantly saying 'What have I done? What have I done?'"
Cobb County Police Sergeant Dana Pierce told CNN on Tuesday that the circumstances of the case had changed significantly since they were first reported.
"I've been in law enforcement for 34 years. What I know about this case shocks my conscience as a police officer, a father and a grandfather," he said.
In a statement issued yesterday afternoon, Cobb County Police Chief John Houser said: "I fully understand the emotional impact this case has on the public, and as concerned citizens and parents everyone wants to know how this could happen."
"In fairness to everyone involved in this emotional case, I would ask that you not make conclusions based on rumor or suspicions and let our judicial system work as it is designed."
Cooper Harris will be buried in a private family ceremony on Saturday.
- Independent
