U.S. Customs and Border Protection

The agency says the teenager was found unresponsive Monday morning during a welfare check at the agency's Weslaco, Texas, station. The teenager's cause of death is unknown.

"The death of a single child in custody of our government is a horrific tragedy," said Jess Morales Rocketto, chair of the advocacy group Families Belong Together. "Four in six months is a clear pattern of willful, callous disregard for children's lives."

U.S. Customs and Border Protection has ordered medical checks on every child in its custody after an 8-year-old boy from Guatemala died, marking the second death of an immigrant child in the agency's care this month.

Just 7 years old, Jakelin Amei Rosmery Caal Maquin was picked up by U.S. authorities with her father and other migrants this month in a remote stretch of New Mexico desert. Some seven hours later, she was put on a bus to the nearest Border Patrol station but soon began vomiting. By the end of the two-hour drive, she had stopped breathing.

Federal authorities say Border Patrol agents have seized nearly 180 pounds (81 kilograms) of marijuana at checkpoints recently set up in New York's Adirondack Mountains.