PHOENIX – A 16-year-old boy diagnosed with the flu died Monday after he was found unresponsive inside a Border Patrol holding area in the Rio Grande Valley in south Texas, U.S. Customs and Border Protection said.

The boy became the fifth Guatemalan minor to die after being apprehended along the U.S.-Mexico border since December.

The border agency identified the teenager as Carlos Gregorio Hernandez Vasquez. He had crossed into the United States illegally on May 13 with a group of more than 70 other migrants near Hidalgo, Texas.

Hernandez Vasquez had spent six days in Border Patrol custody at the central processing facility in McAllen, Texas. In the early hours of Sunday morning, he "self-reported" to staff at the facility that he was not feeling well, a CBP official said in a background call with reporters.

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A contracted nurse practitioner assessed him and determined that the teenager had Influenza A. As part of the treatment, they prescribed him Tamiflu, the official said.

Following his diagnosis, at around midday on Sunday, border agents decided to transport Hernandez Vasquez to the Weslaco Border Patrol station in Weslaco, Texas, to "segregate" him from other migrants and have him continue his treatment, CBP said.

The Guatemalan teenager spent 17 hours to 18 hours at the Weslaco station, during which agents conducted several welfare checks, according to CBP. The final one was an hour before agents found him unresponsive in the temporary holding room inside the station.

Medical staff at the station were unable to revive him, CBP said. Investigators are looking into the cause of death.

"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," CBP acting Commissioner John Sanders said. "CBP is committed to the health, safety and humane treatment of those in our custody."

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CBP added that Hernandez Vasquez was set to be placed at a shelter for migrant youth in Homestead, Florida. But after being diagnosed with the flu, he instead was assigned to a Southwest Key shelter in nearby Brownsville, Texas.

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The agency also said that when he was first apprehended in May 13, Hernandez Vasquez underwent a mandatory health screening. But they noted that at the time it was not recommended that he receive a higher level of care.

In all, the Guatemalan teenager spent nearly seven days in the custody of Border Patrol, far longer than the 72-hour maximum required by law.

A CBP official said each individual minor was handled on a case by case basis, working with the U.S. Department of Health and Human Services' Office of Refugee Resettlement, which takes custody of unaccompanied minors arriving at the border.

In a statement, Mark Weber, a spokesman for the Health and Human Services Department, did not provide details specific to Hernandez Vasquez's case.

But Weber added that HHS has not rejected any referrals for lack of space, and that in the vast majority of the cases, they've been able to find a bed for unaccompanied minors within the 72 hours they are in CBP custody.

"The minority of cases exceeding 72 hours have generally involved exceptional circumstances, such as health issues unique to the UAC (unaccompanied minor), or the placement of a sibling group to ensure they remain together," Weber said.

The deaths of unaccompanied minors

A collection of law enforcement agencies will investigate Hernandez Vasquez' death, looking into medical records while he's in custody and interviewing any other minors who may have been with him in the holding cell, CBP said.

The FBI is among the agencies, the CBP official said.

Hernandez Vasquez is the the fifth minor to die in the United States after being apprehended at the U.S.-Mexico border. All five minors have been from Guatemala.

Most of the deaths have taken place in El Paso or the Rio Grande Valley, which are the two busiest crossing points for illegal entries and for migrants seeking asylum in the United States.

The first reported deaths of minors in the custody of the U.S. government in December drew considerable criticism and prompted the Department of Homeland Security to implement new policies to deal with minors apprehended at the border.

All minors are now required to undergo mandatory medical screenings once they are in U.S. custody. Customs and Border Protection said this has come at a great cost for the agency, financially and in terms of manpower.

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Since Dec. 22, when the new procedures went into effect, CBP has transported an average of 69 migrants a day to hospitals and urgent care centers along the southwestern U.S. border, the agency said.

That has resulted in agents spending nearly 150,000 hours to transport and remain with migrants who are receiving care.

However, these new policies have not prevented the deaths of several minors and adults both in government custody, as well as in hospitals.

The most recent case took place last week, when a 2-year-old Guatemalan boy died in El Paso, after spending several weeks in a hospital.

The other deaths include a 16-year-old boy, a 7-year-old girl, an 8-year-old boy and a 20-month-old girl.

More: Guatemalan migrant, a 16-year-old boy, dies in government custody in Texas

Follow Rafael Carranza on Twitter at @RafaelCarranza.

This article originally appeared on Arizona Republic: Guatemalan teen dies at Border Patrol station, 5th minor to die in US custody in 6 months