The Department of Health and Human Services (HHS) acknowledged Wednesday the previously unreported death of a 10-year-old migrant girl in its custody.

Mark Weber, spokesman for the federal agency that oversees unaccompanied migrant children, said a 10-year-old girl from El Salvador died in the agency's custody on Sept. 29, 2018.

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That means six migrant children have died either in U.S. custody or soon after being released in the last eight months.

Weber said the child had a history of congenital heart defects.

"Following a surgical procedure, complications left the child in a comatose state. She was transported to a nursing facility in Phoenix, Arizona for palliative care in May after release from a San Antonio hospital," Weber said. "On September 26, she was transferred to an Omaha, Neb., nursing facility to be closer to her family. On September 29, the child was transported to Children's Hospital of Omaha where she passed due to fever and respiratory distress."

On Monday, Customs and Border Protection (CBP) confirmed a 16-year-old Guatemalan boy died while in the agency's custody. The cause of death was not immediately known.

Last week, a 2-year-old died after being hospitalized for pneumonia, following a high fever and difficulty breathing.

Sixteen-year-old Juan de León Gutiérrez died in April following "several days of intensive care" at a hospital after falling ill while in the custody of the HHS.

In December, two children, 7-year-old Jakelin Caal Maquin and 8-year-old Felipe Alonzo Gomez, died in custody, prompting a review of CBP policy.

The Congressional Hispanic Caucus ramped up calls on Tuesday for the Trump administration to be more transparent about the deaths of migrant children in its custody.