A Guatemalan teenager has died in U.S. custody, Customs and Border Protection (CBP) confirmed Monday.

The 16-year-old boy was apprehended and processed for illegally crossing the border near Hidalgo, Texas, on May 13, after which he was transferred to the Weslaco Station in the U.S. Border Patrol’s Rio Grande Valley Sector, according to CBP.

The incident marks the fifth death of an immigrant minor at the southern border since December.

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“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 Monday. “CBP is committed to the health, safety and humane treatment of those in our custody.”

After the teenage boy was transferred to the Border Patrol station and slated to be placed in a migrant youth shelter, he was found unresponsive during a welfare check Monday, according to CBP. The case is under review and the cause of death was not immediately known.

The 16-year-old, whose name has not been released by CBP, is the fifth minor to die following apprehension at the border in the past six months. Four of those individuals died in U.S. custody and one died in a hospital.

“Yet another child has died in US government custody," Ashley Houghton, tactical campaigns manager at Amnesty International USA, said in a statement. "This death, which comes days after the administration released a proposal to make it even more difficult for people to seek safety in this country, leads us to wonder how many deaths it will take for the administration to ensure the safety and security of children. There must be an independent investigation into this child’s death.”

CBP has said it expects more than 500,000 migrants to cross the border in fiscal 2019, which began on Oct. 1.

The Department of Homeland Security this month asked the Pentagon for further assistance at the U.S.-Mexico border, by providing support such as tents. Acting Defense Secretary Patrick Shanahan Patrick Michael ShanahanHouse Armed Services chairman expresses confidence in Esper amid aircraft carrier coronavirus crisis Boeing pleads for bailout under weight of coronavirus, 737 fallout Esper's chief of staff to depart at end of January MORE has said U.S. troops will remain at the border until it is deemed secure.

Shanahan previously told Congress that nearly 4,400 troops are stationed at the border and will remain there until September, but said their presence “will not be indefinite.”

Updated at 1:39 p.m.