The boy from Guatemala died shortly after midnight on December 24 at Gerald Champion Regional Medical Center in Alamogordo, New Mexico. The agency said the boy was initially taken to at a New Mexico hospital where he was diagnosed with a common cold, with a fever reaching 103 degrees Fahrenheit, and was prescribed medication.
The agency said in a statement that one of their agents saw the boy had a cough and "glossy eyes" on Monday evening, prompting him to be hospitalized on Christmas Eve.
The agency said the cause of death had not been determined, and that the Department of Homeland Security's inspector-general and the Guatemalan government had been notified.
The official cause of death is not yet known, CBP has said they are conducting an official investigation and will release more information as it is obtained.
Earlier this week, the body of 7-year-old Jakelin Caal, who died earlier this month, was returned to her village in Guatemala for burial. The girl, Jakelin Caal Maquin, had been apprehended in New Mexico after crossing the southern border into the US illegally with her father.
It was the second death of an immigrant child in the agency's care this month. CBP officers and the Border Patrol remain on the job despite the shutdown.
The boy arrived in El Paso with his father December 18 and was transferred to the Alamogordo CBP station on Sunday, according to the statement.
"I was like, 'wow, ' I was shocked", Collman told The Post and Courier of Charleston, describing her call with Trump.
An eight-year-old migrant boy has died in USA government custody.
The previous instance saw seven-year-old Jakelin Caal, also from Guatemala, die just hours after being taken into custody.
He is locked in a battle with Congress over funding for his planned border wall, which he claims will stem migration from Latin American countries plagued by gang violence and poverty.
United States authorities have informed Guatemalan officials of the death.
This photo shows the border fence under construction near New Mexico's Highway 9, near Santa Teresa.
Her death fueled criticism of President Trump's immigration policies from Democrats and migrant advocates.
The child had seizures and a fever, and was flown to the hospital, where she went into cardiac arrest and died.