(CNN) Sen. Elizabeth Warren is demanding an explanation for the recent deaths of five migrant children, writing in a letter Wednesday that the Trump administration owes the public "a full accounting."

"The deaths of five children who had been in CBP care in six months are appalling, and you owe the public an explanation and a full accounting for the causes and circumstances of their deaths," Warren writes in the letter addressed to acting Customs and Border Protection Commissioner John P. Sanders.

Within the last week, two Guatemalan children have died after journeying to the US and being apprehended by authorities -- one died in custody and the other died in a Texas hospital . Andlate last month, a 16-year-old Guatemalan immigrant died at a Texas children's hospital after arriving at an Office of Refugee Resettlement shelter.

The three deaths followed the passing of two young Guatemalans in December: Jakelin Caal Maquin , 7, died of sepsis, a bacterial infection, in an El Paso hospital in December, two days after she and her father were detained. Weeks later, 8-year-old Felipe Gómez Alonzo died of complications from the flu and a bacterial infection while in Customs and Border Protection custody.

In Wednesday's letter, Warren expressed concern about the efforts underway to protect children, noting that after the fatalities in December, then-Homeland Security Secretary Kirstjen Nielsen announced "a series of extraordinary protective measures" to be implemented.

"But the three fatalities this month," Warren writes, "reveal that current efforts to protect children held in U.S. custody after entering the country are failing."

The Massachusetts senator requested answers to a series of questions regarding the deaths, including information on the status of measures announced by Nielsen last December, whether CBP has conducted a review of the three recent fatalities, the specific timeline of those fatalities and whether any other children are known to have died in CBP custody in the past five years.

The answers to those, among others, and a staff-level briefing are requested by June 4.

Warren is not the first member of Congress to call attention to the deaths. On Tuesday, the Congressional Hispanic Caucus demanded thorough investigations into the migrant children deaths in the last six months.

"This episode in our nation's history will be looked upon as a stain on our nation. I would urge all of our colleagues, Republican and Democrat, who care about this issue, who care about the lives of these children, who care about a humane border policy, to join us in calling for an investigation into these deaths," said Democratic Rep. Joaquin Castro, chairman of the Congressional Hispanic Caucus.

CBP said the Weslaco Police Department, the CBP Office of Professional Responsibility, the Department of Homeland Security's inspector general and the FBI are investigating the death of the 16-year-old Guatemalan boy who died in government custody Monday morning.

The death of the Guatemalan teen who died at a Texas children's hospital after arriving to an ORR shelter is also under review.

Immigration authorities have ordered an increase of medical checks for children in their custody, with a focus on children under 10, amid an uptick of migrants crossing the US-Mexico border.

In March, then-CBP Commissioner Kevin McAleenan warned of an impending humanitarian crisis due to a "stark and increasing shift to more vulnerable populations" combined with inadequate capacity for the US government to accommodate the increase in families and children crossing the border.

"The danger of violent assault on that journey, the potential for a tragic incident in the crossing or in overwhelmed CBP facilities, or in transportation networks, is clear and present," he said at the time.

He added that the danger will increase as the weather gets hotter in the summer months.

In April, the Border Patrol arrested 98,977 migrants for illegal entry, many of whom were families, up from the previous month. A CBP official told reporters earlier this week that since December 22, the agency has been transporting about 69 individuals a day to higher-level care facilities, including urgent care and hospitals.