An eight-year-old boy from Guatemala died in US government custody in New Mexico early Tuesday, US immigration authorities have said.
It marked the second death of an immigrant child in detention this month.
US Customs and Border Protection said in a news release that the boy died shortly after midnight.
The death came during an ongoing dispute over border security and with a partial government shutdown underway over President Donald Trump's request for border wall funding.
The White House referred questions to the US Department of Homeland Security, CBP's parent agency.
CBP officers and the Border Patrol remain on the job despite the shutdown.
The agency said the boy showed "signs of potential illness" on Monday and was taken with his father to a hospital in Alamogordo, New Mexico, where he was diagnosed with a cold and a fever.
The boy was prescribed amoxicillin and Ibuprofen and released on Monday afternoon after being held 90 minutes for observation, the agency said.
The boy was returned to the hospital on Monday evening with nausea and vomiting and died there just hours later, US Customs and Border Protection said.
The agency said the cause of the boy's death has not been determined and that it has notified the Department of Homeland Security's inspector general and the Guatemalan government.
The hospital, the Gerald Champion Regional Medical Centre, declined to comment, citing privacy regulations.
CBP promised "an independent and thorough review of the circumstances".
The agency has not yet said when or where the father and son entered the United States or how long they were detained, saying only in its statement that the boy had been "previously apprehended" by its agents.
CBP typically detains immigrants when they cross the border for short periods of time before releasing them or turning them over to US Immigration and Customs Enforcement.
Alamogordo is about 90 miles from the US-Mexico border at El Paso, Texas.
Ruben Garcia, director of El Paso's Annunciation House, said Tuesday that he had no reason to believe his shelter had served the family, but was waiting for further details about what happened.
A CBP spokesman declined to elaborate on Tuesday, but said more details would be released shortly.
A seven-year-old Guatemalan girl died earlier this month after being apprehended by border agents, also in New Mexico.
The body of the girl, Jakelin Caal, was returned to her family's remote village on Monday for burial on Tuesday.
Large numbers of Guatemalan families have been arriving in recent weeks in New Mexico, often in remote and dangerous parts of the desert.
Jakelin and her father were with 161 other people when they were apprehended in Antelope Wells, about 230 miles south west of Alamogordo.
CBP announced new notification procedures in response to Jakelin's death, which was not revealed until several days later.