A 10-year-old El Salvadoran girl with a history of congenital heart defects died in U.S. custody last year, a previously unreported case that brings to six the number of migrant children known to have died after crossing the the southern border in the last eight months, authorities said Wednesday.

Mark Weber, a spokesman for the Department of Health and Human Services, told The Associated Press that the girl died of fever and respiratory distress in September at a hospital in Omaha, Nebraska.

The girl's death was first reported by CBS News.

No other information about the girl was made public, other than that she was in a "medically fragile" state when she entered U.S. care in March 2018.

Five other minors are known to have died in U.S. custody or shortly after having been released since then. All were migrants from Guatemala.

U.S. officials aren't required to disclose such deaths to the public, and as late as Wednesday afternoon, members of the House Homeland Security Committee bombarded acting Homeland Security Secretary Kevin McAleenan with questions about the department's child detention policies on the assumption that five children had died.

"Congress has been more than willing to provide resources and to work with you, Mr. Secretary, to address the security and humanitarian concerns, and, at this point, with five children dead and 5,000 separated from their families, this is intentional," Rep. Lauren Underwood, D-Ill., told McAleenan on Wednesday.

McAleenan replied, "That's an appalling accusation. Our men and women fight hard to protect people in our custody every single day."

On a party-line vote led by Republicans with 10 Democrats absent, the committee voted to strike Underwood's comments from the record as having impugned McAleenan's character.