A 15-year-old Honduran boy is missing after he walked out of the United States' largest migrant children's center Sunday afternoon.

The Casa Padre facility in a former Walmart in Brownsville houses nearly 1,500 boys ages 10 to 17.

Casa Padre workers can't stop children from leaving the shelter, a spokesman for Southwest Key Programs, the nonprofit group that runs the shelter, told Fox News.

"We are not a detention center," Jeff Eller said. "We talk to them and try to get them to stay. If they leave the property, we call law enforcement."

Authorities were called Sunday and asked other groups to help them search the area, which includes a pond, Brownsville police Detective J.J. Treviño told CNN.

The unauthorized and unaccompanied minor had been in the shelter for more than a month, CNN reported. He left during an investigation of a Dallas man's claims that he is the boy's father.

Southwest Key Programs has received more than $807 million in federal grants over the last three fiscal years for supervising immigrant children in 27 facilities along the U.S.-Mexico border.