The size of a US toddler, and the car seat where he perished, could prove crucial to prosecutors who believe his father purposely left Cooper Harris locked in his SUV to die.
Justin Ross Harris asserts that he forgot that his son, obscured by a rear-facing seat, was inside his Hyundai Tucson. Harris says he reported for work and found his son seven hours later while driving to meet friends at a movie theater near Atlanta.
But, based on the line of questioning, investigators apparently suspect that the seat was changed from forward facing to bolster the 33-year-old IT specialist's contention.
Cooper Harris had outgrown the rear-facing seat, Cobb Police Detective Phil Stoddard testified at a probable cause hearing last week.
His parents had purchased a forward-facing seat just six weeks earlier, Stoddard said, but switched back to the old seat "a few weeks" before Cooper's death.
Ross Harris is being held without bond in Cobb County jail on charges of felony murder and second-degree cruelty to children.
His attorney, Maddox Kilgore, has declined all interview requests.
Harris has said his son must have fallen asleep in the short while it took him to drive from a nearby Chick-fil-A - where they had breakfast - to his office.
Once he arrived at Home Depot, Harris drove past an empty space, then backed up into a spot between a parked car and a vacant, grassy area, Stoddard testified at Thursday's probable cause hearing.
Stoddard said Cooper was "several inches" too big for the car cradle, positioned in the middle of the back seat, and testified the child's head would've been visible.
"What would he had to have done to back up?" Cobb Assistant District Attorney Chuck Boring asked.
"He would've had to use rear mirror and both his side mirrors," Stoddard responded, implying Cooper would've been in his father's line of vision.
Surveillance footage showed Ross Harris sitting in the car 30 seconds before reaching behind the passenger seat to grab his computer bag.
At lunchtime, Harris returned to his SUV with a bag of just-purchased light bulbs. According to Stoddard, he approached the car from the left hand side, where they contend he should have noticed his son, who was already dead.
They also say he should have smelled his son's already decomposing body.
A former co-worker, speaking on the on the condition his name be withheld, told The Atlanta Journal-Constitution that Harris had confided to him years ago he had no sense of smell, although there was no mention of that during Thursday's court session.
When she first heard Cooper was in a rear-facing seat, child advocate Janette Fennell of KidsAndCars.org said it made her believe the Harrises were extremely safety-conscious.
"Most parents think you do away with the rear-facing seats after their first birthday, but we now know that's not the best approach," Fennell said.
In 2011, the American Academy of Pediatrics issued a policy statement advising parents to keep their toddlers in those seats until at least their second birthday for safety reasons.
Harris was well-versed on specifics about the car seat, telling detectives he knew the make, model and weight limit.
"When the seat was inspected, the straps for the seat were set ... for a small child," search warrants released Monday state.
Those warrants revealed police are taking a close look at Cooper's health records to see whether the child was, as his father told investigators, developing normally.
"He was walking, talking and appeared to be a normal child for his age," the warrants state.