(CNN) -- It's hard to imagine a young child, strapped into a car seat under a baking sun, slowly succumbing to a hellish death. It's even harder to imagine that it could have been anything other than a terrible accident.
But that's what police in Cobb County, Georgia, say happened last week when 33-year-old Justin Ross Harris left his 22-month-old son, Cooper Mills Harris, strapped into a car seat while he went to work. He's been charged with felony murder and second-degree child cruelty.
Here are the key elements about this disturbing case:
On the morning of June 18, Cooper and his father stopped for breakfast at a Chick-fil-A restaurant near his office in suburban Atlanta. Afterward, Harris put his son into the rear-facing car seat for the half-mile drive to the office. Instead of taking Cooper inside to the day care at his office, police say Harris left the boy strapped into his car seat and went inside to work.
According to police, he came out and opened the driver side door and put something inside at lunchtime. He left the office at 4:16 p.m., stopping a few miles later in a shopping center parking lot, where he called for help, screaming, "What have I done?"
How did Cooper die?
Although final autopsy reports aren't yet in, it appears he died the same way at least 619 other children have died since 1998, according to a study by a San Francisco State University researcher: his body got too hot and there wasn't anyone there to cool him down. It's a condition called hyperthermia.
"Essentially what happens is that the body temperature gets so high that it begins to kill the cells," forensic pathologist and former medical examiner Vincent DiMaio told HLN. "It's like you're cooking something."
While hyperthermia can strike anyone with enough exposure to heat, babies and small children strapped into rear-facing car seats are at particular risk, if a parent or caregiver forgets they are there. On an 80-degree day, a car can heat to an unsafe temperature in just two minutes, according to the National Weather Service, and the temperature can rise to 123 degrees in just one hour.
On the day Cooper died, the temperature peaked at 88 degrees.
So why is the father charged with murder?
Police at first seemed sympathetic to what seemed like a tragic accident, but later said there was more to the case.
"The chain of events that occurred in this case does not point toward simple negligence and evidence will be presented to support this allegation," Cobb County police Chief John R. Houser said in a statement.
A subsequent affidavit released by the Magistrate Court of Cobb County may indeed support the allegation. The document states that when investigators interviewed Justin Harris, "he stated that he recently researched, through the Internet, child deaths inside vehicles and what temperature it needs to be for that to occur. Justin stated that he was fearful this could happen."
What are his family and friends saying?
Leanna Harris spoke in defense of her husband during Cooper's funeral Saturday in Tuscaloosa, Alabama, Justin Harris' hometown. Justin Harris spoke as well, by phone from jail, thanking attendees "for everything you've done for my boy." The family has not answered questions from the media. Neither has anyone close to the family. Of the numerous family and friends of Harris contacted by CNN, none would speak. Most said they had been advised not to.
Cooper's obituary says the boy was "loved and cherished and protected by both parents and all family members for his short 22 months of life." And on a now-closed change.org petition calling on authorities to drop the charges, several posts from people who said they know Harris spoke well of him. "He has been nothing but a caring father and supporting husband," wrote one poster, identified as Michael Gordon of Northport, Alabama.
Justin Harris is being held without bond at the Cobb County jail after pleading not guilty last week. He's scheduled for a preliminary appearance on July 3.
Because he's charged with murder, a grand jury will also have to review his case if the magistrate in an upcoming hearing finds probable cause for the charge to stand.
CNN's Nick Valencia and Mary Lynn Ryan contributed to this report.