COBB COUNTY, Ga. -- A 22-month-old boy left in a hot car for several hours died of hyperthermia, according to an autopsy report.

Cobb County police issued a new warrant against Justin Ross Harris on Wednesday. Harris is accused of killing his 22-month-old son, Cooper, by leaving him in a hot car for over seven hours.

Harris is being charged with felony murder and second-degree cruelty to a child. Originally, Harris was charged with felony murder and first-degree cruelty to a child.

According to the Cobb County Medical Examiners' Office, the boy's cause of death was determined to be from hyperthermia, which is consistent with a death from hot temperatures.

"Investigative information suggests the manner of death is homicide," reads a statement released by the Cobb County Police Department.

WARRANT: Read the criminal warrant for Justin Harris

Harris told police he'd forgotten to drop his son off at daycare that morning.

The warrant states Harris, 33, placed his son into a rear facing car seat after eating breakfast with his son at the Chick-fil-a on Cumberland Boulevard. Harris left his son in the car when he entered his workplace.

RELATED: Tips to prevent hot car injuries and deaths

During Harris' lunch break, the warrant states, he returned to the car through the driver's side door to place an object in the vehicle. At around 4:16 p.m., Harris left work in his vehicle and pulled over at a shopping center on Akers Mill Road asking for assistance with his child.

Police arrested and charged Harris with child cruelty and felony murder a little over five hours after he discovered his son's body.

Cobb County Chief of Police John Houser issued a statement on Wednesday saying, in part:

I understand that tragic accidents similar to this one do occur and in most cases the parent simply made a mistake that cost them the life of their child. This investigation, although similar in nature to others, must be weighed on its own merit and the facts that lead our detectives to charge the father must be presented at the appropriate time during the judicial process. The chain of events that occurred din this case does not point toward simple negligence and evidence will be presented to support this allegation.

It's the first time police have publicly stated that Cooper Harris' death was not an accident and not a mistake.

"Let the judicial system handle what it's supposed to handle," Cobb County Police Officer Mike Bowman said in Wednesday afternoon news conference. "Let us do our job, let us get the information out there, don't be so quick to judge."

The news did not come as a surprise to at least two witnesses who watched Ross Harris' reaction when he pulled into a parking lot after allegedly realizing his son was dead.

"It seemed like he was acting. If you ask me, I say so," said Raphael Hayes.

Several witnesses watched as Ross Harris paced back and forth, while a passerby performed CPR on his son.

"This guy was running all over grabbing the trees yelling, 'What have I done, what have I done," said Ed Cockerham. "Wasn't near his son. If that would have been my kid, I would have been right there holding the baby."

Other witnesseshave stated that Harris' pain seemed real.

Harris has a probably hearing scheduled for July 3. He remains jailed without bond.

The funeral for Cooper Harris will be held in Alabama on Saturday.

STATEMENT: Read Cobb Co. Police Chief John Houser's letter here