PHOENIX -- Autopsies on two small Arizona children left strapped in car seats for hours in a closed vehicle have determined the infant girl and her toddler brother died from exposure, and they suffered no trauma and had no drugs in their bodies. The children's 20-year-old mother, Brittany Velasquez, was arrested and charged in the deaths shortly after the bodies were discovered in the vehicle.

The reports by the Pinal County Medical Examiner's Office say the March 26 deaths of 2-year-old Lorenzo Velasquez and 9-month-old Brooklyn Velasquez in the isolated Arizona community of Superior were "consistent with exposure to the elements."

The reports signed by Medical Examiner Dr. John Hu were released late Monday in response to a records request by The Associated Press.

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Authorities with the Pinal County Sheriff's Office have said Velasquez was the last person to see her children alive when she left them in a car outside a family home at about 9:30 a.m. on March 26 and went to work. The children were dead when Velasquez returned to the car nearly 14 hours later. They were still in the car seats and wearing the same clothes they had on in the morning.

The autopsies say sheriff's officials reported Velasquez left the baby in a parked car "for a prolonged period of time" at least two times in the past.

Velasquez in April pleaded not guilty to two counts of second-degree murder and child abuse charges in the deaths of her children in Superior, a hardscrabble mining town of about 2,900 people some 60 miles east of Phoenix. The vehicle was parked outside the home of relatives.

The National Weather Service says the temperature in Phoenix's Valley of the Sun region reached 75 degrees on March 26. The autopsy report consulted with the private service Weather Underground, which gave a slightly lower high temperature reading for 71 degrees in the area that day.

"It is NEVER safe to leave a toddler, disabled person or pet locked in a car," National Weather Service says on its web page.

The advocacy group KidsAndCars.org says temperatures inside vehicles can shoot up much higher very quickly and estimates an average of 37 children die in hot cars each year in the United States.

Velasquez initially had said someone else was supposed to be watching the children that day, but sheriff's officials said there was no evidence of that.

A pre-trial conference in the case has been set for Aug. 10 in Pinal County Superior Court.