WASHINGTON (Reuters) - A 16-year-old Guatemalan boy died on Monday in U.S. Border Patrol custody in Texas, U.S. officials said, making him the fifth Guatemalan minor to die after being apprehended at the U.S.-Mexico border since December.

The boy, Carlos Hernandez, was apprehended by U.S. Border Patrol agents on May 13 after crossing the border illegally near Hidalgo, Texas, with a group of 70 others, according to U.S. Customs and Border Protection, which oversees the Border Patrol.

Early on Sunday morning, Hernandez told staff at the central processing station where he was being held that he was not feeling well, a CBP official told reporters. He was diagnosed with the flu and transferred to the Weslaco Border Patrol Station in south Texas later that day to separate him from others at the processing station in the Rio Grande Valley, the official said.

He was due to be transferred to the custody of the Department of Health and Human Services, which oversees the care of minor migrant children who cross into the United States without adult family members, the official said.

But on Monday morning, during a “welfare check,” the boy was found unresponsive, according to a CBP statement. The statement said the cause of death was not yet known, and that the Department of Homeland Security’s watchdog and the Guatemalan government had been notified.

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

The Guatemalan foreign ministry requested that U.S. authorities urgently explain the cause of death. Local and federal law enforcement are investigating Hernandez’ death, the CBP official said.

The boy was the fifth Guatemalan minor since December to die after being apprehended at the U.S.-Mexico border. Four of them died while in U.S. custody. A fifth child, who crossed the border with his mother in April, died this month after weeks in the hospital, but had already been released from U.S. custody at the time of his death.

Record numbers of families from Central America are traveling to the U.S.-Mexico border and asking for asylum in the United States, fleeing poverty and violence in their home countries. From October 2018 through this April, nearly 293,000 unaccompanied children or people traveling in families were apprehended at the southern U.S. border - nearly four times the number during the same period the prior year.

That has in turn strained U.S. border facilities, which are the first stop for migrants after they are detained. Reuters photos taken last week showed adults and children outside the U.S. Border Patrol station in McAllen, Texas, sleeping on the ground and rigging up makeshift awnings with reflective blankets to shelter form the sun. One Guatemalan man told Reuters that he and his 9-year-old son had spent nearly two weeks in Border Patrol custody in Texas, sometimes sleeping on the ground.

Since Dec. 22, CBP has transported about 69 people a day to higher level of care facilities, including urgent care and hospitals, the official said.

The Trump administration has asked Congress for $4.5 billion in immediate emergency funding, which would represent a 44% increase in spending for programs that house, feed, transport and oversee the migrants.

But immigrant advocates say the administration’s policies, including making it more difficult for migrants to seek asylum at official ports of entry, contribute to making their journeys more arduous and drive migrants to seek out remote border outposts badly equipped to care for children.

Julie Linton, co-chair of the American Academy of Pediatrics’ Immigrant Health Special Interest Group, said she was concerned about sick children potentially being housed in bare-bones Border Patrol facilities for extended periods of time.

“There certainly need to be conditions that do not include lying on a mat with a Mylar blanket on a floor that is cold, and cage-like fencing that extends to the ceiling,” she said on a conference call with reporters on Monday. “We absolutely need pediatric health experts at the border.”