The 8-year-old Guatemalan migrant who died this week in the custody of US Customs and Border Protection had the flu, according to the New Mexico Office of the Medical Investigator.
An official cause of death for Felipe Gomez Alonzo has not yet been determined.
The boy, who was detained with his father, died shortly before midnight Christmas Eve at Gerald Champion Regional Medical Center in Alamogordo, New Mexico, about 90 miles north of the border crossing in El Paso, Texas.
An autopsy on the boy shows he tested positive for influenza B, the medical examiner's office said Friday.
Felipe was taken to the hospital Monday after a border agent noticed signs of illness, and the medical staff first diagnosed him with a common cold and later detected a fever.
"The child was held for an additional 90 minutes for observation and then released from the hospital mid-afternoon on December 24 with prescriptions for amoxicillin and Ibuprofen," CBP said in a statement. Amoxicillin is a commonly prescribed antibiotic.
On Monday evening, the boy began vomiting and was taken back to the hospital for evaluation. He died hours later, the CBP said.
Felipe was the second Guatemalan child to die in US border patrol custody this month.
On December 8, Jakelin Caal Maquin, 7, died in a hospital two days after she was taken to a Border Patrol station.