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.
What is known about this case?
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.
Speaking to CBS News, Democratic Texas congressman Joaquin Castro accused the government of covering up the death.
"It's outrageous that another child has died in government custody and that the Trump administration didn't tell anybody," said Mr Castro, who on Tuesday spoke of an "epidemic of death" in the border area.
Writing on Twitter after the girl's death was announced, Mr Trump said: "Democrats don't want to fix the loopholes at the border. They don't want to do anything. Open borders and crime!"
What about the other cases?
On Monday, 16-year-old Carlos Gregorio Hernández Vásquez from Guatemala died at a detention centre in Texas. An official cause of death has not been announced.
Customs and Border Protection (CBP) acting commissioner John Sanders said the agency was "saddened by the tragic loss of this young man" and that they were "committed to the health, safety and humane treatment" of those in custody.
The Congressional Hispanic Caucus, a group of 38 Democratic members, demanded a federal investigation into the child deaths and detention on the border.
What is happening at the border?
More than 300,000 people were apprehended at the US-Mexico border between January and April, according to official statistics, with numbers rising every month.
Officials say they do not have the facilities to handle the rising numbers of migrants, mainly from Guatemala, Honduras and El Salvador. Many migrants say they are fleeing persecution, poverty and violence in their countries and plan to seek asylum in the US.
Opposition lawmakers, rights groups and international charities have condemned the treatment of people at the border, pointing to cramped holding pens and shoddy accommodation.