Officials revealed on Wednesday that a 10-year-old migrant girl died in U.S. custody last year, making her the first of now six such deaths in the past eight months.

The girl, who was from El Salvador, died in U.S. Department of Health and Human Services (HHS) custody on Sept. 19, 2018, agency spokesperson Mark Weber told CBS News. Her death was not previously reported.

Weber said the girl, whose identity has not been revealed, had a history of congenital heart defects and was in a “medically fragile” state when she entered an HHS Office of Refugee Resettlement (ORR) facility in San Antonio, Texas, on March 4, 2018.

“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 [2018] after release from a San Antonio hospital,” Weber told CBS in a statement. “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.”

Officials are required to inform local child welfare authorities and report the cases internally, but they are not required to publicly announce the deaths. HHS did not immediately respond to HuffPost’s request for comment.

The girl was the first death of a migrant child in federal custody since 2010, according to officials. She is the first of now six migrant children since September 2018 to die in federal custody — the five other children were from Guatemala.

Carlos Hernandez Vasquez, 16, died on Monday after a week in Border Patrol custody. Officials said the teen was brought to a Border Patrol station in Weslaco, Texas, after he told personnel at the McAllen migrant detention center that he felt sick. Border Patrol operates under Customs and Border Protection, a branch of the U.S. Department of Homeland Security.

Border Patrol said Tuesday that the McAllen center suspended intake of new migrants because it identified many people in custody who displayed signs of influenza. The facility resumed normal intake operations as of Wednesday afternoon.

On May 14, a toddler apprehended at the border with his mother died after spending several weeks in a Texas hospital. CBP said the family was no longer in Border Patrol custody at the time of the 2 ½-year-old boy’s death.

On April 30, 16-year-old Juan de Leon Gutierrez died after he was reported sick at a Texas youth detention facility. Doctors later found he had a rare condition called Pott’s puffy tumor, which is often caused by a severe sinus infection.

On Dec. 24, 8-year-old Felipe Gomez Alonzo died in U.S. custody from what officials believed to be the flu. On Dec. 8, 7-year-old Jakelin Caal Maquin died in Border Patrol custody of dehydration and shock.

CBP has said it is working to provide migrants the best care possible amid the growing number of families crossing the border. But children keep dying in Border Patrol custody, and medical experts and immigrant rights advocates warn that it won’t stop, considering the difficult journey migrant children take to get to the U.S. and the unsanitary conditions in Border Patrol centers.

National Center for Youth Law immigration director Neha Desai previously told HuffPost that the government is “jeopardizing the health and lives of children in their custody” until it begins “complying with its basic legal obligations.”

On Wednesday night, former Rep. Beto O’Rourke (D-Texas) called the 10-year-old girl’s death a “cover up,” and demanded an investigation into both her death and why it was only discovered now.