But officials at the agency were unable to determine with certainty the whereabouts of 1,475 children, Mr. Wagner said.
The new details come as Congress is examining safeguards put in place by the Department of Health and Human Services and the Department of Homeland Security to make sure children who show up alone at the border are turned over to relatives, and not human traffickers.
Senator Rob Portman, Republican of Ohio and the chairman of the subcommittee, said he was shocked that government officials did not know the whereabouts of hundreds of migrant children released from their care.
“The fact that nearly 1,500 kids are missing in just a three-month period is simply unacceptable,” he said. “H.H.S. has a responsibility to better track these children so they aren’t trafficked or abused, and so they show up to their court hearings.”
Two years ago the subcommittee released a report detailing how health and human services officials placed eight children with human traffickers who forced the minors to work on an egg farm in Marion, Ohio.
That report found that department officials had failed to establish procedures to protect the unaccompanied minors, such as conducting sufficient background checks on potential sponsors and following up with sponsors to ensure that the children were safe. As a result, the children were turned over to the people who contracted them out to the egg farm.
To prevent similar episodes, the Homeland Security and Health and Human Services Departments signed a memorandum of understanding in 2016, and agreed to establish joint procedures within one year for dealing with unaccompanied migrant children.
More than a year after the new guidelines were due, the two agencies have not completed them, Mr. Portman said.
Children who show up at the border by themselves are usually apprehended by Border Patrol agents or turn themselves in to customs officers at the Department of Homeland Security. Once they are processed, they are turned over to the custody of the Department of Health and Human Services’ refugee office. The office runs more than 100 shelters around the country where it houses children and provides care until they can be turned over to a sponsor while awaiting their immigration hearings.
The sponsors are usually parents or family members already residing in the United States. The sponsors are supposed to undergo a detailed background check, including making sure they have not been convicted of crimes.
After the children have been placed with sponsors, workers at the department follow up with calls to ensure that the minors continue to live with the sponsors, are enrolled in school and are aware of their court dates.
But several immigration advocates who work with unaccompanied children said the department did very little follow-up. Allison E. Herre, a lawyer with Catholic Charities of Southwestern Ohio, said she had seen sponsors who forced the children to work instead of attending school and who failed to ensure that the children attended their immigration court proceedings.
Since 2016, health and human services officials said they have started calling sponsors to check on children 30 days after placement. But the agency said it was not legally responsible for children after they had been released from its refugee office.
Mr. Wagner, the health and human services official, said the agency was re-examining its interpretation of existing laws to make sure that migrant children were not turned over to smugglers or human traffickers.