The official, Steven Wagner, the acting assistant secretary of the agency’s Administration for Children and Families, disclosed during testimony before a Senate homeland security subcommittee that the agency had learned of the missing children after placing calls to the people who took responsibility for them when they were released from government custody.

The children were taken into government care after they showed up alone at the Southwest border. Most of the children are from Honduras, El Salvador and Guatemala, and were fleeing drug cartels, gang violence and domestic abuse, government data shows.

From last October to the end of the year, officials at the agency’s Office of Refugee Resettlement tried to reach 7,635 children and their sponsors, Mr. Wagner testified. From these calls, officials learned that 6,075 children remained with their sponsors. Twenty-eight had run away, five had been removed from the United States and 52 had relocated to live with a nonsponsor.

But officials at the agency were unable to determine with certainty the whereabouts of 1,475 children, Mr. Wagner said.