"The chain of events that occurred in this case does not point toward simple negligence, and evidence will be presented to support this allegation," said Cobb County Police Chief John House.
A criminal warrant released Wednesday described the events that led to Cooper's death.
A timeline of events
On the day Cooper died, Harris stopped for breakfast at a fast-food restaurant and afterward strapped his son into a rear-facing child restraint seat on his SUV's back seat, police said.
He drove to his workplace, a Home Depot corporate office, about a half-mile away. He works as a Web designer there.
Usually, he would take his son to an on-site day care. But that day, police said, Harris left him in the car seat.
During his lunch break, he returned to his car, opening the driver's side door to put something inside, police said.
After work, around 4:16 p.m., the 33-year-old father got in his car and drove away. A few miles away, he stopped the car at a shopping center and called for help.
When it became clear Cooper was dead, Harris was so inconsolable police had to restrain him.
"What have I done?" he wailed as he tried to resuscitate the boy.
A wave of sympathy
Each year, dozens of children die from heatstroke in cars, according to KidsandCars.org. More than 40 died last year. The organization says its tally is likely incomplete and much lower than the real toll.
The charging of Harris triggered a wave of sympathy and a vigorous debate over whether the heartbroken father should be punished.
Two change.org petitions urging authorities to release Harris were started and then shut down this week. One petition posted this note: "I think that based on the recent developments this petition is no longer relevant. I still pray that this was truly an accident. If that is the case, the DA now knows that the community does not want Justin prosecuted on murder charges."
Another, set up at YouCaring.com, has raised more than $22,000 for the Harris family.
"Please don't listen to the media. It just upsets me to watch it," wrote Heather McCullar, who set it up. "Please don't listen to the media. The family will speak when they can."
Contacted by CNN via e-mail, she wrote back, "No one is allowed to comment right now."
'The manner of death is homicide'
As Harris sits in jail, his wife, Leanna, would not discuss the case with the media.
The Cobb County medical examiner's office found the child's cause of death "consistent with hyperthermia and the investigative information suggests the manner of death is homicide," according to a Cobb County Department of Public Safety statement issued Wednesday. Temperatures hit 92 degrees Fahrenheit on the day of his death.