The Associated Press reports that an autopsy on two children left in a car show that their death was a result from exposure to heat. In Phoenix, Arizona, the two kids were left strapped for hours in their car seats in a closed vehicle. The infant girl and her toddler brother did not have any drugs in their systems and did not show any signs of trauma. They both died from heat exposure.

The two children died on March 26, and the Medical Examiner released the autopsy results on Monday night.

Brittany Velasquez, 20, the children’s mother, was arrested and is now being charged in the deaths. She was the last person to see the children alive when she left them in the car outside of a family home at 9:30 a.m. while she went to work. She returned almost 14 hours later to find the children dead, still in their car seats.

Velasquez pleaded not guilty to two counts of second-degree murder and child abuse. She claimed she thought someone was going to watch her children that day, but police have been unable to confirm that statement. The car was parked outside of her relatives’ home.

*Photo credit The Associated Press