An eight-year-old Guatemalan boy has died while in the custody of US Customs and Border Protection (CBP).
The child, who has not yet been named, is the second migrant child to have died in US detention this month.
He had been in CBP custody on Christmas Eve when a US border patrol agent noticed he appeared unwell, and was transferred to a medical centre where he was diagnosed with a cold and fever, according border officials.
He was released shortly later with prescriptions for amoxicillin and ibuprofen, but later began suffering from nausea and vomiting and was readmitted to hospital.
The child, who was travelling with his father, died shortly after midnight on Christmas Day.
The exact cause of his death is not known.
The news comes as the body of Jakelin Caal, a 7-year-old girl who died earlier this month while being apprehended by border agents, was returned to her family's village in Guatemala.
Relatives said the youngster was excited at the prospect of starting a new life thousands of miles away from the remote community where she had grew up, an area decimated by deforestation.
The child, one of 163 people to have handed themselves over to border officers, died of dehydration and shock in a hospital in El Paso, Texas.
A CBP statement said the US had seen a "dramatic increase" in arrivals of children and families arriving "illegally or without authorisation" who were held at federal facilities.
"During their period of detention they received medical screenings and further treatment as needed," the release said.
Thousands of migrants from central America have travelled to the US border, hoping for a better life as they flee violence and poverty in their home countries.
Border force officials in Texas regularly deal with large groups of people, including many children, crossing the border.
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The detention of children, who were previously held apart from their parents in border area camps, has been subject to particular criticism.
President Donald Trump has said he wants to keep migrants out, suggesting people crossing the border illegally should not be able to apply for asylum.