Soon after 10-year-old Darlyn Valle crossed the southern border into the U.S. last year, personnel with the Border Patrol determined she had a debilitating heart condition.

Valle had somehow made the 1,400 mile journey from her native El Salvador to Texas, according to Carlos Martinez, president of Asociacion Cívica Salvadoreña de Nebraska, an Omaha nonprofit group that helped arrange legal and postmortem assistance for the girl's family. Valle's name was confirmed to CBS News by the girl's mother.

Valle was just over halfway through the trek from El Salvador to Omaha, where her mother lives, when Border Patrol apprehended her in March.

But she didn't make it to Omaha until September 26. Officials brought her there so she could spend her final three days near her mom.

"Unfortunately right now, the facilities that we have, the resources we've been afforded, and the legal framework that we operate under is not sufficient, is absolutely overwhelmed and it's insufficient. And that continues to be an ongoing challenge," said Robert Perez of Customs and Border Patrol.

Mark Weber, a spokesperson for the Department of Health and Human Services, said in a statement to CBS News Wednesday that the agency oversaw the medical care for a dying 10-year-old girl last year. He said the girl had a history of congenital heart defects. Her death was previously undisclosed to the public.

Martinez's group was founded in 2012, and helps families send bodies of deceased loved ones to El Salvador. Martinez said the government of El Salvador helped pay for the return of Valle's body.

Valle's death was the first of a migrant child in federal care since 2010, according to federal officials.

In the eight months since Valle' death, five children from Guatemala have died either in U.S. custody or soon after being released from custody to a hospital. They ranged in age from 2 1/2 to 16.

The wave of deaths has led lawmakers to call for an investigation into the medical practices of Department of Homeland Security and the Office of Refugee Resettlement.

Rep. Raúl M. Grijalva, a Democrat from Arizona, blamed the White House for the rash of deaths. "As Trump plays politics with asylum-seeking children and their parents, the results are increasingly deadly — with six children dying in U.S. custody since September 2018," Grijalva said in a press release Thursday.

In an interview with CBS News Wednesday night, Congressional Hispanic Caucus Chair Rep. Joaquin Castro accused the administration of concealing the girl's death, and said he wanted the government to turn over more information. "We're going to make immediate inquires to HHS to find out what happen to this young girl," Castro said.

Congresswoman Rosa DeLauro, chair of the subcommittee that funds the Department of Health and Human Services (HHS), said in a statement that she would "ensure accountability and transparency."

"It is outrageous that Trump administration officials waited nearly eight months before releasing any information," the statement read. "What would drive these officials to cover up such a serious matter?"

Officials are required to notify local child welfare authorities and report such deaths internally, but are not required to announce them to the public.