US Customs and Border Protection announced medical checks on all children in its custody following the death of an eight-year-old Guatemalan migrant -- the second child fatality in American detention this month.
Guatemala called for an investigation into the boy's death and some US lawmakers angrily denounced the Trump administration's immigration policy.
The boy, identified by Texas congressman Joaquin Castro as Felipe Alonzo Gomez, had been transferred to a New Mexico medical center showing signs of sickness on Monday, the agency said Tuesday.
Staff diagnosed him with a cold but later discovered a fever. He was released at midday, with prescriptions for ibuprofen and the antibiotic amoxicillin.
Alonzo, who was with his apprehended father, was later transferred back to the hospital suffering from nausea and vomiting. He died on Monday, just before midnight.
The CBP said it had not established an official cause of death but would "ensure an independent and thorough review of the circumstances".
Late Tuesday, commissioner Kevin K. McAleenan announced the agency was "conducting secondary medical checks upon all children in CBP care and custody," and "reviewing its policies with particular focus upon care and custody of children under 10.
"This is a tragic loss. On behalf of US Customs and Border Protection, our deepest sympathies go out to the family," he added.
McAleenan said the CBP was considering seeking medical support from other government agencies, among them the US Coast Guard and the Department of Defense.
"CBP is coordinating with the Centers for Disease Control on the numbers of children in custody as well," he said.
Guatemala called on US authorities to conduct a "clear" investigation of the death, adding that "medical reports have been requested... to clarify the cause of death of the child".
The news of the boy's death triggered outrage among lawmakers.
"The Administration's policy of turning people away from legal ports of entry ... is putting families and children in great danger," Castro, chairman elect of the Congressional Hispanic Caucus, said in a statement.
"Another child dies under this Administration's watch," tweeted Democratic Congressman Marc Veasey, also of Texas.
"Such a devastating story to hear on Christmas Day."
"Heartbroken and sickened by this news," Senator 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," he tweeted.
- 'Searching for a dream' -
The boy's death came on the same day that Jakelin Caal, a Guatemalan girl who died in US custody under similar circumstances earlier this month, was buried.
Her body was repatriated on Sunday and after a long journey reached San Antonio Secortez, the remote village where her family -- members of the indigenous Q'eqchi' Maya people -- live without electricity and other basic services.
"This girl left home happy searching for a dream, but unfortunately died on the way," community leader Alberto Pop told AFP in the cemetery.
"You hear that in the United States they pay well, not like the companies here in Guatemala -- that's why people leave," said Pop, whose 22-year-old son Joaquin left in November in search of the American dream.
"I don't know if he is alive or dead."
Caal's December 8 death reignited debate in the United States over immigration policy and the treatment of migrants.
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 wall along the border with Mexico, which he says 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."