A 16-year-old Guatemalan boy who died Monday in immigration custody in south Texas was diagnosed with the flu a day before, a Customs and Border Protection official said. The teenager is the fifth migrant child to die in U.S. custody since December.

Carlos Gregorio Hernandez Vasquez was found unresponsive Monday morning during a welfare check at Weslaco Border Patrol Station, a CBP official familiar with the case said in a Monday afternoon teleconference.

He had been transferred to the station Sunday from the Rio Grande Valley Sector’s Central Processing Center, the official said.

Early Sunday, Carlos told staff at the processing center that he was not feeling well and a nurse practitioner assessed him and determined he had influenza A, the official said. Carlos was prescribed the medicine Tamiflu for treatment, which Border Patrol agents picked up from a local pharmacy, the official said.

The official said Carlos continued to receive care at the processing center throughout the day and was transferred midday to the Weslaco station, where he was to be segregated from much of the other migrant population due to his illness. At the station, he was again medically assessed and his medication was turned over to the medical professionals there, the official said.

The next morning, Carlos was found unresponsive in one of the station's short-term holding rooms about an hour after his previous welfare check, the official said. Medical staff were unable to treat him or save his life, the official said. He had been in the border patrol station for about 17-18 hours at the time.

When asked why the boy was not taken to a hospital for treatment, the official said such a decision was up to the medical care providers at their facilities. The official added that while they did not have the individual specifics on the symptoms Carlos was exhibiting at the time, the official suspected it was one of the matters that would be reviewed in the investigation.

The Weslaco Police Department, CBP’s Office of Professional Responsibility, the Department of Homeland Security's Office of Inspector General and the FBI are investigating the incident, the official said.

Carlos was apprehended by McAllen station agents after crossing the border May 13 near Hidalgo, Texas, the official said.

Carlos was due to be transferred to the custody of the Department of Health and Human Services, which oversees the care of unaccompanied or separated migrant children after they are initially processed by immigration authorities.

The boy was initially set to be placed in the shelter for migrant children in Homestead, Florida, but because of his illness was then given placement in the Southwest Key Casa Padre Shelter in Brownsville, Texas, the official said Monday afternoon.

“The men and women of U.S. Customs and Border Protection are saddened by the tragic loss of this young man and our condolences are with his family,” acting CBP Commissioner John Sanders said in a statement. “CBP is committed to the health, safety and humane treatment of those in our custody.”

The Guatemalan government has been notified, the statement said.

Guatemalan officials said in a statement that the boy had family in the United States.

The boy is the fifth migrant child to die since December. All of the children were Guatemalan. Asylum-seekers and other migrants from Guatemala have been fleeing a mix of violence, drought, food shortages and poverty.

In December, Jakelin Caal Maquín, 7, died of a bacterial infection, and Felipe Gomez Alonzo, 8, died of complications from the flu and an infection while in CBP custody, according to authorities.

On April 30, Juan de León Gutiérrez, 16, died following "several days of intensive care" at a hospital after falling ill while in the custody of the Department of Health and Human Services.

A medical examiner in Corpus Christi, Texas, said Juan had been diagnosed with a rare condition known as Pott’s puffy tumor, which can be caused by a severe sinus infection or head trauma, according to The Associated Press.

Last week, a 2½-year-old died after being hospitalized for pneumonia, following high fever and difficulty breathing after he was apprehended in early April, authorities said.