Top Democrats on the House Judiciary Committee are demanding the Department of Homeland Security (DHS) open an investigation into the deaths of five migrant Guatemalan children who died either in Customs and Border Protection (CBP) custody or soon after.

The deaths raise "significant questions about the conditions in CBP's short-term holding facilities, the length of time children are held in such facilities, and the general suitability of such facilities for families and children," Rep. Jerry Nadler, the committee chairman, wrote in a letter to Acting DHS Secretary Kevin McAleenan on Friday.

The letter, which was cosigned by three other committee members, cites CBS News reporting about 16-year-old Carlos Hernandez Vásquez, who was held by CBP for seven days before his death on May 20, despite rules requiring unaccompanied migrant children be transferred to the Office of Refugee Resettlement (ORR) with 72 hours.

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Officials said the boy reported feeling ill on his sixth day in custody at the CBP processing center in McAllen, Texas. He was treated with Tamiflu and brought to a different Border Patrol facility, but by the next morning was unresponsive.

In the days following his death, the McAllen facility was briefly shut down as more than 30 others displayed flu-like symptoms.

"Due to the seriousness of this tragedy and the many questions that remain, we request you initiate an investigation into this incident, as well as the four other deaths in CBP custody over the last few months," the members of Congress wrote.

The U.S. government has seen an unprecedented string of deaths of migrant children ranging from 2.5 to 16 years old. All have been attributed to illnesses. In the five most recent cases, dating back to December, the children were in custody only briefly before dying.

On May 22, CBS News revealed the earliest such case. Ten-year-old Darlyn Valle died in the custody of the ORR on Sept. 29, 2018. Officials said she was apprehended six months earlier and had a pre-existing heart condition.

Prior to Valle's death, no migrant children had died in federal custody since 2010, according to officials with DHS and ORR.

Valle's case was not mentioned in the lawmakers' letter to McAleenan. However, on Saturday the Congressional Hispanic Caucus sent a letter to DHS and the Department of Health and Human Services demanding more information about the case.

DHS did not reply to a request for comment.

The legal nonprofit Refugee and Immigrant Center for Education and Legal Services (RAICES) lauded the Judiciary Committee's call for a DHS investigation in a statement to CBS News, but called for an independent inquiry.

"It would be completely insufficient for the same people who presided over these deaths to investigate wrongdoing done on their watch," said Erika Andiola, chief of advocacy for RAICES.