The Department of Health and Human Services (HHS) lost track of nearly 1,500 migrant children that it placed with sponsors in the United States, a top official with HHS told Congress on Thursday.

This announcement raised concerns that the unaccounted-for children could end up in the hands of human traffickers, or be used for labor by people posing as relatives, the New York Times reports.

The children—most fleeing gang violence and domestic abuse in Honduras, El Salvador, and Guatemala—were taken into government care after arriving alone at the Southwest border.

Between last October and the end of the year, HHS's Office of Refugee Resettlement attempted to reach 7,635 children and their sponsors. According to the testimony of Steven Wagner, the acting assistant secretary of the agency's Administration for Children and Families, most children's whereabouts were confirmed. However, officials found that 28 children had run away, five had been removed from the U.S., and 52 had relocated to live with a non-sponsor. That left 1,475 children unaccounted for.

Federal officials came under fire two years ago after retracting child protection policies for minors fleeing violence in Central America. On Thursday, senators said that HHS had failed to take full responsibility for the children's care and had delayed reforms needed to keep them from falling into the hands of human traffickers.