SUPERIOR, Ariz. — The mother of two small children found dead after being left strapped in car seats in a vehicle tried to blame their deaths on someone else who was supposed to be watching them, authorities said Wednesday.

A narrative of events released by the Pinal County sheriff's office says there is no proof for 20-year-old Brittany Velasquez's story that she dropped the children off with someone who was to care for them. That person's name is blacked out in the document.

Velasquez is being held on $2 million bond on suspicion of two counts of first-degree murder in the Monday deaths of her 2-year-old son and infant daughter in Superior, a hardscrabble mining town of about 2,900 people some 60 miles east of Phoenix.

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Authorities say 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. Monday 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.

Brittany Velasquez, left, and her two children, Christopher, 2, and Brooklyn, 10 months KPHO

A cause of death for the children has not been determined.

The National Weather Service says the temperature in the region reached 75 degrees on Monday. "It is NEVER safe to leave a toddler, disabled person or pet locked in a car," the National Weather Service says on its web page.

The advocacy group KidsAndCars.org estimates an average of 37 children die in hot cars each year in the United States.

"There was no proof that Brittany actually dropped the children off," the document said. "It is known that the kids spent several hours in the vehicle as there was condensation on the inside windows of the vehicle and the children were cold to the touch."

At her court appearance on Tuesday, the attorney representing Velasquez pointed out apparent inconsistencies in the police narrative, specifically around whose car the children were strapped into and whose car Brittany took to work - hers or her grandmother's.

10-month-old Brooklyn Velasquez, left, and 2-year-old Christopher Velasquez, right Amber Velasquez via KPHO

The judge agreed "it is confusing," but "we know that the kids were fine in the morning and then they were dead at 11 o'clock at night, and they'd been in that car for some period of time."

The probable cause statement said Superior Police officers were called to the scene Monday night by Velasquez, who reported finding her two children unresponsive in a vehicle in front of the house. She told them the children were not breathing and blood was coming from their mouths.

They were pronounced dead at the scene.

The unidentified person Velasquez tried to blame for the children's deaths denied there was any arrangement for her to care for them, authorities said.

The other person "stated she had no idea what was happening and did not know what Brittany was talking about," according to the sheriff's document.

A Superior Police Department report from January cited Velasquez's grandmother Sally Velasquez saying Velasquez left the toddler and infant with her for days at a time.

Superior police called Arizona child protection officials in early January because of those concerns about the children after the grandmother claimed Velasquez had stolen a $3,500 fur coat.

But the coat was later returned and the grandmother did not seek prosecution.

The police report also indicated that in that incident, Velasquez would not be investigated for child neglect or abuse unless the grandmother stated she was no longer willing to care for the children.

In response to a request from CBS News' Crimesider about previous contact with the family, a spokesman for the Department of Child Safety said in an email Velasquez was investigated in October 2016 and January 2018, both times on reports that she was leaving the children with her grandmother.

In both cases, the agency said, they found no evidence to suggest the children were being abused or neglected. It said case workers determined Velasquez had a job and that the apartment where she then lived in the city of Mesa had everything needed for her children.

"We understand these types of tragic events evoke emotional reactions; we too feel pain when children suffer. However, we can only make decisions based on the available evidence and what the law allows," the agency said in a statement released to Crimesider. "The Department acted in good faith based on the information we received and exercised our due diligence during these prior investigations."

Pinal County Superior Court Administrator Todd Zweig said Velasquez would be represented by the county public defender's office. A specific attorney has not yet been assigned to her case. Velasquez could not be reached at the jail.

Autopsies were being conducted to determine causes of death of the children. Autopsy results could take months, the sheriff's office said.