U.S. Customs and Border Patrol patrol along the border fence, June 27, 2018, in Hidalgo, Texas.
U.S. Customs and Border Patrol patrol along the border fence, June 27, 2018, in Hidalgo, Texas.

A Guatemalan boy in U.S. Customs and Border Protection custody died Monday, a day after beginning treatment for the flu. 

Carlos Gregorio Hernandez-Vasquez, 16, was found "unresponsive" while being held at the Weslaco Border Patrol Station

U.S. officials have not determined the cause of death, which is generally issued after an investigation by multiple federal agencies.

Hernandez-Vasquez complained he was not feeling well on Sunday morning at CPB's Rio Grande Valley Central Processing Center where he was being held.

FILE - Detainees are seen inside fenced areas at R
FILE - Detainees are seen inside fenced areas at Rio Grande Valley Central Processing Center in Rio Grande City, Texas, June 17, 2018.

A health check by a nurse practitioner that day indicated he had Influenza A. He was prescribed Tamiflu, an antiviral drug, a CBP official familiar with the case told reporters on a phone call Monday.

He was then sent to the Weslaco station on Sunday to "segregate" him and await transfer to a facility for minors, the official said.

Staff last checked on Hernandez-Vasquez an hour before he was found dead.

If the boy was sick a week earlier when officials detained him on May 13 near Hidalgo, Texas, after entering the country unlawfully, he showed no signs of illness severe enough to prompt a higher level of care, the official said. 

CBP said that after being held in two facilities for detained migrants, the teen was set to be transferred to the custody of the Office of Refugee Resettlement, the agency within the U.S. Department of Health and Human Services tasked with caring for unaccompanied children who cross the border.

U.S. law requires unaccompanied children who aren't from Canada or Mexico generally be transferred from border or immigration officials to HHS custody within 72 hours.

Asked why Hernandez-Vasquez was not transferred on the proscribed timeline, the CBP official told reporters that HHS officials had determined when and where he would be transferred. Why HHS took a week to sort out the boy's placement remains unclear.

While he was originally going to be sent to a facility in Homestead, Florida, HHS instead decided to send him to one in Brownsville, Texas, to shorten travel times for the sick teen. 

The boy was able to call his family twice during custody, the CBP official added. The last time was May 16. 

Hernandez-Vasquez is the fifth Guatemalan child to die in U.S. official custody at the border since December. His death comes as U.S. border officials are detaining an increasing number of unaccompanied minors and families crossing the southern border.

That figure climbed from roughly 4,968 children in October to nearly 8,897 in April; and from 23,116 families to 58,474 families in the same period, leaving officials scrambling to provide adequate services.