The United States government risks placing migrant children in the custody of human traffickers because federal agencies have delayed crucial reforms needed to keep the children safe, according to the findings of a Senate subcommittee obtained by The Associated Press.

Federal officials came under fire two years ago for rolling back child welfare policies meant to protect unaccompanied minors fleeing violence in Central America, and lawmakers said Thursday that the agencies had yet to take full responsibility for the children's care in the US.

Since the dramatic surge of border crossings in fall 2013, the federal government has placed more than 180,000 unaccompanied minors with parents or other adult sponsors in communities nationwide, where they are expected to attend school while they seek legal status in immigration court.

An AP investigation found in 2016 that more than two dozen unaccompanied children had been sent to homes across the country where they were sexually assaulted, starved, or forced to work for little or no pay. At the time, many adult sponsors didn't undergo thorough background checks, government officials rarely visited homes and in some cases, had no idea that sponsors had taken in several unrelated children, a possible sign of human trafficking.

Since then, the Health and Human Services Department (HHS) has used its limited funding to boost outreach to at-risk children deemed to need extra protection, and last year offered post-placement services to about one-third of unaccompanied minors, according to the Senate Permanent Subcommittee on Investigations.

But advocates say it is hard to gauge the total number of minors who may have been exposed to dangerous conditions, in part because some of the migrants designated for follow-up disappear before social workers reach them, and sponsors can refuse a house call.

"HHS has an obligation to better track these kids and ensure they show up to their court hearings because the potential for trafficking and abuse remains an issue," said Sen. Rob Portman (R) of Ohio, chair of the subcommittee, which will hold a follow-up hearing Thursday.

Senator Portman began investigating in response to a case in his home state of Ohio, where eight Guatemalan teens were placed with human traffickers and forced to work on egg farms under threats of death. Six people have been convicted and sentenced to federal prison for their participation in the trafficking scheme that began in 2013.

The hearing comes as the Trump administration has called for amending a law to allow the government to send more migrant children back to their home countries more quickly if they are not at risk of human trafficking. The administration also is pushing to terminate the settlement of a class-action lawsuit that ensures unaccompanied minors are housed in the "least restrictive" setting, preferably with their parents or other adult relatives, while they await hearings in immigration court.

In June, the administration announced it would begin arresting sponsors who had hired smugglers to bring their children into the US, a move that sent a shudder through immigrant communities nationwide.

HHS declined to respond to AP questions about the subcommittee's findings.

The Department of Homeland Security has coordinated the care, custody, and removal of unaccompanied minors with its counterparts at HHS, according to a copy of written agency testimony obtained by the AP.

"DHS has worked closely with the Trump administration and members of Congress to address existing 'loopholes' that allow individuals to exploit our immigration laws," said written testimony by James McCament, a DHS deputy undersecretary. "This not only safeguards the child, it protects the integrity of our immigration system."

Once migrant children turn 18, they are no longer eligible to be held in facilities run by HHS, and the agency is required to let DHS know whether the children should be detained or released into the community. But HHS only forwards those plans for one of every three children, the subcommittee found.

The number of children seeking refuge in the US has not returned to the height of the surge starting in 2013, but spiraling gang and drug violence in Honduras, Guatemala, and El Salvador continue to push children, teens, and families to migrate northward.

The panel's top Democrat, Sen. Tom Carper of Delaware, said the agencies need to do more to prevent abuse.

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"It is clear that we are failing to adequately protect these children, many of whom have fled horrific violence, experienced trauma, and now face significant challenges in their new homes," Senator Carper said in a statement. "The government agencies responsible for overseeing these children must be doing more to address the systemic shortcomings that this committee has already identified."

This story was reported by The Associated Press.