An eight-year-old migrant from Guatemala died in government custody on Tuesday, US Customs and Border Protection said, the second child fatality in US detention this month.
The boy, who was with his apprehended father, had been transferred to a New Mexico medical center showing signs of sickness on Monday, the agency said.
Staff diagnosed him with a cold but later discovered a fever. He was released at midday, given prescriptions for ibuprofen and the amoxicillin antibiotic.
The child was later transferred back to the hospital after showing signs of nausea and vomiting, dying just after midnight.
The CBP said it had not established the cause of death but would "ensure an independent and thorough review of the circumstances."
Guatemala called on US authorities to conduct a "clear" investigation of the death, also saying that "medical reports have been requested... to clarify the cause of death of the child."
The news of the boy's death triggered outrage on social media, days after the body of a Guatemalan migrant girl who died in similar circumstances was returned home.
Her death reignited debate in the United States over immigration policy and the treatment of migrants.
"Another child dies under this Administration's watch," tweeted Democratic Congressman Marc Veasey of Texas.
"Such a devastating story to hear on Christmas Day."
"Heartbroken and sickened by this news," Martin Heinrich of New Mexico wrote on Twitter.
"I am urgently demanding more details, but the Trump administration must be held accountable for this child's death and all the lives they have put in danger with their intentional chaos and disregard for human life," Heinrich tweeted.
President Donald Trump has made hardline immigration policies a central plank of his presidency, drawing fire from critics who accuse him of demonizing migrants for political gain.
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.
"Heartbroken to hear of a second child's death in CBP custody," tweeted Nydia Velazquez, a Democratic congresswoman from New York.
"We must demand accountability, find answers and put an end to this Administration's hateful, dangerous anti-immigrant policies."