(CNN) A 16-year-old Guatemalan immigrant has died in US custody, days after arriving at an Office of Refugee Resettlement shelter.

The boy died Tuesday at a Texas children's hospital where he had been treated for several days in the intensive care unit, according to a written statement from Evelyn Stauffer, spokeswoman for the Department of Health and Human Services' Administration for Children and Families.

"The cause of death is currently under review and, in accordance with standard ORR policies and procedures, the case will be subject to a full review," Stauffer said.

Stauffer's statement, first reported by BuzzFeed , does not identify the boy or detail the government facilities where he'd been held. He's described as an "unaccompanied alien child," the term officials use for migrant children who arrive in the United States without parents or guardians.

A source close to the situation said the boy showed signs of distress the day after he arrived at Casa Padre , a former Walmart in Brownsville, Texas, that's one of the nation's largest shelters for unaccompanied minors.

The boy was taken to a local hospital twice and eventually airlifted to a children's hospital in Corpus Christi, Texas, the source said.

Guatemala's foreign ministry said Wednesday the boy had undergone an emergency operation at the hospital after presenting with a severe infection in his frontal lobe. The infection did not improve, the foreign ministry said, even after surgery to relieve pressure in the boy's skull.

The boy's death comes months after the deaths of two other immigrant children who fell ill in government custody: 7-year-old Jakelin Caal Maquin and 8-year-old Felipe Gómez Alonzo , who both died in December in the custody of US Customs and Border Protection.

Those cases spurred sharp criticism from immigrant rights advocates over the care children receive while in custody. They also prompted congressional hearings and a decision by immigration authorities to increase medical checks of children in custody

Statements detail days of medical care

The 16-year-old who died this week was originally from Camotán, Guatemala, according to Guatemala's foreign ministry.

Statements released by US and Guatemalan officials on Wednesday detailed the days leading up to his death:

• On April 19, according to Guatemala's foreign ministry, the boy crossed the US border near El Paso, Texas, and was detained by the Border Patrol.

• On the evening of April 20, he was transferred to the ORR shelter, Stauffer said. Before his arrival, no health concerns were observed by CBP clinicians, Stauffer said, and he "did not note any health concerns" when he arrived at the shelter.

• On April 21, Stauffer said, the boy became "noticeably ill including fever, chills and a headache." Shelter personnel took him to a hospital emergency room that morning. He was treated and released back to the shelter later that day, Stauffer said.

• On the morning of April 22, after his health hadn't improved, he was taken to another hospital emergency department via ambulance. "Later that day the minor was transferred to a children's hospital in Texas and was treated for several days in the hospital's intensive care unit," Stauffer said.

• The boy underwent emergency surgery at that hospital, Guatemala's foreign ministry said. On April 30, after days of 24-hour intensive care, he died there.

Officials don't know whether the child had a preexisting condition or got sick during his journey from Guatemala, said Silvia Samines, vice consul at the Guatemalan Consulate in McAllen, Texas.

Shelters house thousands

ORR is the arm of the government tasked with providing temporary shelter to undocumented immigrant children who arrive in the United States alone. Last year the office's shelters also housed thousands of children who were separated from their parents as a result of the Trump administration's zero tolerance policy.

Children are held in a network of more than 100 shelters in 17 states while officials work to identify sponsors who can care for them as their immigration cases make their way through the system. The average length of stay for an unaccompanied child in the program was 89 days during the first quarter of this fiscal year, according to government statistics

Authorities have long maintained they're committed to protecting children in their custody.

More than 49,000 unaccompanied minors were referred to ORR shelters last year. In March, one HHS official told CNN the office was preparing for the number of children in its care to double

Details about this case are still emerging, but already immigrant rights advocates are arguing the boy's death shows the dangers of a system that puts children's lives at risk. And on Twitter, the Congressional Hispanic Caucus called on the Department of Homeland Security and the Department of Health and Human Services to investigate.

Guatemala's government said Wednesday that it was helping the boy's family with paperwork and had offered financial support to cover the cost of repatriating the boy's remains.

"The Guatemalan government regrets the death of this young Guatemalan and offers its condolences to the family, reiterating that it will be providing the corresponding support in the process of repatriation," the foreign ministry said.