A 10-year-old girl from El Salvador died in US custody last year, a previously unreported case that brings to six the number of migrant children known to have died after being detained by border authorities in eight months.
The girl had a history of congenital heart defects and died in September at a hospital in Nebraska, officials say.
Her name, when and how she had entered the US have not been disclosed.
Meanwhile, Democrats have demanded an investigation into the deaths.
President Donald Trump has vowed to clamp down on illegal immigration at the US-Mexico border amid an increase in the number of people trying to cross the frontier in recent months.
He has repeatedly criticised Democrats for their opposition to his plans on immigration and declared a national emergency in February to bypass Congress and appropriate funds for a border wall.
The girl entered the care of an Office of Refugee Resettlement facility in San Antonio, Texas, in a "medically fragile" state on 4 March 2018, Mark Weber, a spokesperson for the Department of Health and Human Services (HHS), said in a statement. The HHS provides care to unaccompanied minors.
Following an unspecified surgical procedure, complications left her in a comatose state, he added. After her release from hospital in May, the girl was sent to a nursing facility in Phoenix, Arizona, for palliative care.
On 26 September, she was transferred to a nursing facility in Omaha, Nebraska, to be "closer to her family", Mr Weber said. She died three days later due to "fever and respiratory distress".
Officials are not required to announce the deaths to the public. This case marks the first death of migrant child in federal custody since 2010.