One of the boy's brothers discovered his body Monday morning, the Las Vegas Review-Journal reported. Police Lt. Ray Steiber said the family returned from a trip Sunday afternoon and the little boy was apparently overlooked.

A man who said he belonged to the same Mormon congregation as the parents told the newspaper the 4-year-old was mentally disabled. He was not strapped in a car seat when found.

Christine Skorupski, a spokeswoman for Clark County Family Services, said all of the family's minor children were placed temporarily in a shelter.

Investigators were waiting for autopsy results to determine if the boy died from heat exhaustion.

At least five children have died in hot cars in the United State this year.