The day his son died, Harris was planning to see "22 Jump Street" with friends after work, according to Stoddard.
Detective: He swore at police officers at the scene
Harris never called 911 and said "f*** you" to a police officer on the scene who asked him to get off his phone, the detective said.
Stoddard said that Harris told police he couldn't reach anyone on his telephone, but phone records show he made three calls after he discovered his son's body, and one between him and his employer lasted six minutes.
Defense believes he was forgetful
Defense attorney Maddox Kilgore said it's not unusual for people to forget children in cars.
"It's easy to get distracted when you get behind the wheel. Everyone's done it. His mind is already skipping ahead to the rest of the day," Kilgore said.
"The results of that forgetting ... were absolutely catastrophic," he said. "But an accident doesn't become a crime because the results were catastrophic."
Investigator: Couple had financial, intimacy issues
Harris and his wife had $2,000 and $25,000 life insurance policies on their son, according to the investigator. He also said Harris' wife "was complaining about (her husband's) sporadic purchasing or overcharging credit cards."
Harris' wife told police the pair were having intimacy issues, according to Stoddard. Texts indicate that she knew he was cheating on her, he said.
In warrants obtained by CNN on Friday, police said Harris discussed with family members the procedure for cashing a life insurance policy, but there is no other information or time element.
"Through the investigation Harris has made comments to family members regarding a life insurance policy that he has on Cooper and what they need to do in order to file for it," the warrants say.
His Internet searches are a focus of case
Harris accessed sites about "people who die," which showed videos of suicides, executions, war, etc., police said. He also visited a site on life without a child and did searches on surviving prison and on "age of consent for Georgia," they said.
"We've only scratched the surface," Stoddard said of the searches.
His activities on day of son's death questioned
The day Cooper died, the father and son stopped at Chick-fil-A for breakfast before he drove the 0.6 miles to work. The breakfast was not out of the ordinary for them.
"Justin stated that this happens two or three times a month. It's daddy-son time, a special occasion to them," Stoddard said.
When he arrived at work, surveillance footage shows Harris backed up his car, according to Stoddard. He said Harris didn't have a backup camera and would have had to turn around or use his rearview and side mirrors. Investigators used a mannequin in the car seat and found the boy's head would have been visible over the top of the car seat.