Sen. Elizabeth Warren took aim Tuesday at the "appalling" string of deaths of migrant children in U.S. custody and demanded to know what plans the Trump administration has to protect those detained at the southern border.

"I have been and continue to be extraordinarily concerned about this administration's treatment of immigrant children," the Massachusetts Democrat wrote.

Her questions came in a letter (pdf) to John P. Sanders, Acting Commissioner of U.S. Customs and Border Protection (CBP), dated Tuesday—one day after the death of a 16-year-old Guatemalan boy who was in government custody.

He was fifth migrant child to die since December after being detained.

That child, Carlos Gregorio Hernández Vásquez, "had been held by immigration authorities for six days—twice as long as federal law generally permits—then transferred him to another holding facility even after he was diagnosed with the flu," The Associated Press reported.

Warren, in her letter, noted that the teen's death came less than a week after the death of a two-year-old who'd migrated with a parent and had been detained by border authorities. The toddler died after weeks spent in the hospital with pneumonia.

The heartbreaking list goes on: 16-year-old Juan de León Gutiérrez died on April 30. Two younger children died in December 2018—seven-year-old Jakelin Caal Maquin and eight-year-old Felipe Gómez Alonzo.

"The deaths of five children who had been in CBP care in six months are appalling," wrote Warren, "and you owe the public an explanation and a full accounting for the causes and circumstances of their deaths."

Warren referenced her trip last year to the immigration processing facility in McAllen, Texas, where she said she "witnessed children being held in appalling conditions by CBP at the height of the Trump administration's family separation policy."

She pointed to the DHS Inspector General's review of the December 8, 2018 death of seven-year-old Jakelin, but said its completion date is unknown. In the meantime, wrote Warren, lawmakers and the public need to know protections are in place.

The senator also gave a June 4 deadline to a series of questions about the deaths.

"Children are dying," wrote Warren, "and CBP must do more to end this string of tragedies."

Amnesty International also expressed concern following 16-year-old Hernández Vásquez's death Tuesday.

"This death, which comes days after the administration released a proposal to make it even more difficult for people to seek safety in this country," said Ashley Houghton, tactical campaigns manager at Amnesty International USA, "leads us to wonder how many deaths it will take for the administration to ensure the safety and security of children."