The government agency tasked with placing thousands of Central American children into communities while they await immigration court decisions has no system for tracking the children, does not keep complete case files and has allowed contractors to operate with little oversight, according to a report released Monday by the Government Accountability Office.

“Based on the findings in this report, it’s no wonder that we are hearing of children being mistreated or simply falling off the grid once they are turned over to sponsors,” said Sen. Charles E. Grassley (R-Iowa). “The Obama administration isn’t adequately monitoring the grantees or sponsors whom we are entrusting to provide basic care for unaccompanied children.”

Three senators — Grassley, Orrin G. Hatch (R-Utah) and Tom Coburn (R-Okla.) — asked the GAO in October to review policies of the federal Office of Refugee Resettlement. The agency provides shelter for unaccompanied minors fleeing violence in Central America and identifies sponsors to care for them while they await hearings in immigration courts. More than 125,000 unaccompanied minors from Central America have been caught at the U.S.-Mexico border since 2011. The 64-page report is being released one day before the Senate Judiciary Committee is scheduled to hear testimony from Obama administration officials about their handling of the children.

“Their records are incomplete, they are not appropriately checking in on the facilities that house the children, and they don’t even have a dedicated system to follow up on the children once they’ve been placed with sponsors,” Grassley said.

The Office of Refugee Resettlement, a division of the Department of Health and Human Services, has come under criticism in recent weeks for its handling of a number of cases involving unaccompanied minors.

Advocates for unaccompanied minors say that the refu­gee office was overwhelmed by the surge of children crossing the border in 2014 but that the system is a much better alternative than longer detention for vulnerable children.

On Jan. 28, the Senate Permanent Subcommittee on Investigations issued a report focusing on cases in which Central American children were victims of abuse by their sponsors, including one case where the agency released several Guatemalan teenagers to labor traffickers who forced them to work long hours at an Ohio egg farm for as little as $2 a day.

“We agree with the GAO’s recommendations, which is why we’ve already implemented some of them and are in the process of implementing the rest,” said Andrea Helling, a spokeswoman for the Department of Health and Human Services. “This is part of the process of improving the program to care for the children who come into our custody.”

The GAO found that children’s case files were often incomplete, making it difficult for investigators to determine whether they had received proper care such as group counseling and clinical services. Investigators reviewed 27 randomly selected children’s case files. None of them contained all of the required documents.

The report also criticized the agency’s oversight of nonprofit groups that it pays to operate shelters for the children and locate sponsors. In 2014, the agency implemented a new monitoring process, requiring site visits every two years. However, investigators found that the agency didn’t complete the site visits in 2014 and 2015. In 2014, agency staff members visited 12 of 133 sites. By August 2015, they visited 22 of 140 sites.

These monitoring visits revealed several problems at the nonprofit-run shelters. At one site, agency workers discovered that the facility didn’t give children the proper amount of medication, leading them to accidentally overdose.

Helling said the Office of Refugee Resettlement is aware of the issues and has hired additional staff and implemented new policies to ensure that all site visits are completed in fiscal 2016.

Once children are released to sponsors, the agency has no system for tracking their whereabouts, according to the report. Some children, including those who have been identified as trafficking victims, are supposed to receive services such as mental- health care. In fiscal 2014, only 9.5 percent of children released by the agency received these services. The agency has established a call center for children who want to report problems with their sponsors and requires its caseworkers to call all children and sponsors after the children are placed.

Grassley sharply criticized the lack of follow-up for released children.

“Beyond the risks to the children created by these shortcomings, our communities are left to cope with the crime and violence from gang members and other delinquents who are not identified or tracked because of HHS’s haphazard and porous practices,” he said.

Helling said the agency is looking at ways to expand post-release services for children, adding that “the overwhelming majority of these children are fleeing violence and chaos, not looking to create it.”

Sen. Rob Portman (R-Ohio), who co-chaired the Jan. 28 Senate hearing about problems within the agency, said he will testify at Tuesday’s hearing.

“I’m pleased the Judiciary Committee is following up on the subcommittee’s bipartisan investigation,” he said. “The administration must be held accountable for turning young children over to traffickers and criminals.”

Jennifer Podkul, a migrant rights expert at the Women’s Refugee Commission, said: “Overall, we’re incredibly happy that ORR is the agency that’s been designated to release the kids. What happened when there were incredible numbers was that it showed the strain and the weaknesses in the system. It was like a magnifying glass on the system.”

VanSickle is a reporter for the Investigative Reporting Program, a nonprofit news organization at the University of California at Berkeley.