Migrant families walk from the Rio Grande, which separates the United States and Mexico in Texas, before being apprehended by Border Patrol officials in March. A Guatemalan teenager who was taken into custody in April fell ill and died. (Eric Gay/AP)

A 16-year-old Guatemalan boy who crossed the U.S.-Mexico border without his parents fell ill while in federal custody and died Tuesday after spending several days in intensive care at a hospital in Texas, U.S. Health and Human Services said Wednesday.

The teenager, who was not identified, had cleared initial health screenings after encountering U.S. officials at the border. But he deteriorated shortly after he arrived at the Casa Padre shelter, a converted Walmart housing approximately 1,400 minors in the border city of Brownsville, Tex. Officials have not released a cause of death, and the case remains under investigation.

Guatemala’s Foreign Ministry said in a statement Wednesday that the teen had “a severe infection” in his brain and underwent emergency surgery at a Corpus Christi hospital before he died. Guatemalan officials said the boy was from the Camotan municipality in the southeastern state of Chiquimula and crossed the border into the El Paso area on April 19.

“The government of Guatemala laments the death of this young Guatemalan and offers its condolences to the family,” the statement said. Officials are working to repatriate the teen’s remains to Guatemala.

The boy was the third migrant child to die in federal custody in the past five months, as a soaring number of Central American families and unaccompanied minors have been trying to enter the United States via the southern border.

Advocates for immigrants called for an immediate investigation into the death, which was disclosed on the day that the White House sought emergency funding from Congress to address the border influx, including billions of dollars in humanitarian aid that could increase HHS shelter capacity from nearly 13,000 migrant youths to more than 23,600.

HHS spokeswoman Evelyn Stauffer said Customs and Border Protection clinicians examined the teenager and cleared him for transfer to the HHS shelter. CBP officials declined to say when he crossed the border or how long he was in agency custody before he was transferred to the shelter. When he arrived at the shelter on the night of April 20, the teen “did not note any health concerns,” officials said.

The next morning, the teen fell “noticeably ill,” with a fever, chills and a headache. Shelter workers took him to the emergency room at a nearby hospital, which they did not identify. He was treated and released to the shelter later that day.

Still sick on April 22, the teenager was taken by ambulance to a second hospital emergency room. He was then airlifted to Driscoll Children’s Hospital in Corpus Christi, where he spent about a week in intensive care and died, according to a person with knowledge of the case who spoke on the condition of anonymity to discuss the health care of a migrant.

BuzzFeed was first to report the death Wednesday. Southwest Key Programs, which runs the Casa Padre shelter, declined to comment and referred questions to HHS.

The Nueces County medical examiner’s office and a spokesman for Driscoll Children’s Hospital said they did not have information about the cause of death.

Carlos Holguín, a lawyer involved in a long-running class-action lawsuit regarding the care of migrant children, called the incident “troubling.” He said he immediately asked a court-appointed monitor in the case to investigate.

“We’re gravely concerned for the health and safety of the children,” he said.

Holguín said the death was the first he had heard of in HHS custody. In December, two young Guatemalan children died in Customs and Border Protection custody: Felipe Gomez Alonzo, 8, died of complications from influenza B infection, and Jakelin Caal, 7, died from a bacterial infection.

HHS said the Guatemalan teenager’s brother and consular officials were allowed to visit him in the hospital in recent days, and hospital staff updated his relatives in Guatemala about his condition.

On Wednesday, the White House sent Congress a $4.5 billion emergency spending request for humanitarian aid and border security, saying the border influx is “threatening lives.”

A record number of Central American families and unaccompanied minors are surrendering to U.S. authorities at the border in large groups — including one this week with more than 400 people in New Mexico — and the number of unaccompanied children sent to HHS custody this year is 50 percent higher than the same period last year. Dozens of children and adults have been discovered suffering from various illnesses and have been taken to local hospitals, some with preexisting health concerns, including influenza and liver disease.

About $2.8 billion in the emergency request would go toward HHS’s Office of Refugee Resettlement, which oversees a network of shelters where minors are sent after Customs and Border Protection processes them at the border.

HHS is supposed to house the unaccompanied minors in what are considered less restrictive settings — it is not supposed to be akin to jail — and to place them with a parent or guardian quickly, a process that now takes about two months.

Erica Werner and Nick Miroff contributed to this report.