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Austin Police are calling 3-month-old Carylton Stewart's death an accident.
The Medical Examiner's officer says it was the heat that killed him: Hyperthermia, or overheating.
APD says the boy was found inside a white Sebring at about 2:45 Thursday afternoon.
Employees at the Waffle House on Ben White Boulevard pulled him out of the unlocked car and called 911 but it was too late.
Dr. Catherine Yee with the Dell Children's E.R. says intense heat eventually starts to damage the brain.
"Once your body hits a certain temperature, the proteins that make up all of our body stop working well. And proteins are in charge of the majority of our body functions, so your body really can't do anything past a certain temperature. Your brain doesn't work, none of your organs work," Yee said.
A separate incident -- In Round Rock on Wednesday, 40-year-old Rebel Gough was arrested for leaving her 4-year-old and 5-year-old in a car that was on but the A/C was blowing hot air according to police paperwork.
Thankfully those kids are okay.
Stewart Williams is with "Safe Kids Austin." They work to educate parents on this deadly trend.
"It's a break in their routine and they forget that they have a child with them," Williams said.
Williams encourages parents to create reminders.
"Leave something in the back seat with them that you need when you get to your destination. That might be your cell phone, that takes care of texting and driving also...but leave your cell phone in the backseat with them, leave your purse back there, your wallet. Take a shoe off and put it in the back seat if you need to," Williams said.
Captain Rick Rutledge with Austin-Travis County EMS says when your car is not being used, make sure it's locked and your children can't get to it. Rutledge remembered a call he went out on.
"The parents thought the child was in another room. Turned out they were in the car and it was too late before anybody knew it. So make your car inaccessible when you're not driving it," Rutledge said.
Lt. Andre Delareza with the Austin Fire Department says if you see a child trapped in a hot car, the first step is to call 911. They'll be there in 4 minutes or less.
"If the individual inside, the baby or the animal looks stressed, we're going to pop that window, we're going to break it. We have tools that are specially designed for that," Delareza said.
AFD uses a tool called a window punch. He says trying to break the window yourself isn't easy.
"It's not like Hollywood tells us, you can't just punch open the window. You're going to break yourself. It's easy to hurt yourself or possibly aggravate the injuries to the child inside the car," Delareza said.
"Before you start swinging clubs or doing anything to break glass, try the doors. If a window is partially open, yank it out," Rutledge said.
But if a citizen absolutely has to break the glass, AFD says don't break the window closest to the child.
"If you're going to do this yourself, don't go for the center of the window, go for one of the corners of the window. That's going to make it safer," Delareza said.
APD says since the death of Stewart has been ruled an accident, no charges have been filed and no arrests have been made.
But the investigation is still open -- information will be presented to a grand jury.