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.