The Maricopa County Medical Examiner has named the cause of death for a 9-year-old migrant child who died at a hospital in Mesa after crossing the border with his mother last month.

In an initial report, medical officials say the child died from hyperthermia, which is when the body overheats, and environmental heat exposure.

In a statement about the death, Customs and Border Protection says on the night of June 15, the Santa Cruz County Sheriff's Office patched over a 911 call to the Border Patrol's Tucson Sector about a female migrant and her two children. The mother told 911 operators her son was having seizures and needed medical help. The National Weather Service shows a high temperature of 97 degrees that day. 

Agents and first responders with the Tubac Fire Department took the child to a hospital nearby that night, according to CBP.  He was airlifted to Banner Desert Medical Center in Mesa the next afternoon, where he was placed on life support with multi-organ failure. He died the next night on June 17. 

A spokesperson for Maricopa County says the case is still open as the medical examiner’s office reviews the child’s medical records, but an autopsy won’t be performed. The case is also under investigation by CBP's Office of Professional Responsibility. A CBP spokesperson said the mother and her other child were no longer in Border Patrol custody but declined to answer further questions.