NEW BLOOMINGTON, Ohio — On the phone, the boy was frantic. After traveling hundreds of miles from a village in Guatemala, he had made it across the U.S. border and into a government-funded shelter for unaccompanied minors.

But then something went terribly wrong.

Instead of sending him to his uncle, Carlos Enrique Pascual, a landscape worker in Florida, authorities said the shelter released the teenager to traffickers who took him to central Ohio, held him captive in a roach-infested trailer and threatened to kill him if he tried to leave.

“Please, how can I get out of this?” Pascual’s nephew begged him during a stolen moment with a telephone. “I’m hungry, and my heart is bursting with fear.”

Pascual called police and, in December 2014, authorities found his nephew, then 17, and seven other boys living in cramped, dirty trailers about an hour outside of Columbus. Authorities said they were working at Trillium Farms, one of the country’s largest egg producers, debeaking hens and cleaning cages nearly 12 hours a day, six days a week, for as little as $2 a day.

The boys were part of a surge of children flowing across the U.S.-Mexico border over the past four years, overwhelming federal officials responsible for their safekeeping, child advocates say. Since 2011, more than 125,000 unaccompanied minors from Central America have been stopped at the border, many placed in shelters funded by the federal Office of Refugee Resettlement.

Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) has demanded a response from the Obama administration to whistleblower claims that thousands of those children have been released to sponsors with criminal records that include homicide, child molestation and human trafficking. Legal advocates for the children say many have wound up in abusive situations, where they have been forced to work to repay debts or living expenses. Some children simply stop showing up for immigration hearings and vanish.

“We have a large percentage of these kids that disappear, and I don’t know what happens to them,” said Jessica Ramos, a lawyer with Advocates for Basic Legal Equality, an Ohio nonprofit group that represents children in immigration proceedings.

Andrea Helling, a spokeswoman for the Department of Health and Human Services, which includes the Office of Refugee Resettlement, said the inspector general is investigating the whistleblower allegations. She acknowledged that the agency briefly relaxed identity requirements for family members collecting children at the height of the surge in May 2014 to help place children more quickly.

Since then, she said, the agency has strengthened its protection efforts by reinstituting a fingerprint requirement for many people who claim children from federally funded shelters, expanding a hotline to report abuse, and requiring caseworkers to call and check up on children within 30 days of their release.

“We are committed to placement of unaccompanied children with appropriate sponsors that serve the best interest of the child,” Bob Carey, the agency’s director, said in a statement.

Still, the agency conducted post-release checks on only about 6,500 children in fiscal 2014, Helling said. Once the children are settled with sponsors, she added, state and local child protection agencies are responsible for their well-being.

“Once a child is placed with a sponsor,” she said, “the local community becomes very important.”

Helling declined to discuss the agency’s handling of individual children, including Pascual’s nephew. Federal prosecutors indicted six people in connection with the trafficking scheme; five have plead guilty.

No legal action has been taken against Trillium Farms, whose executives say they were unaware that a subcontractor hired to provide manual labor was engaged in human trafficking.

“Our employment guidelines are strict, and we participate in all federal programs to verify employment,” chief operating officer Doug Mack said in a statement. “While we have the same requirements for our contractors, it is clear in this case we were misled by the contracting company, which intended to act illegally.”

Alarmed by the case, Sen. Rob Portman (R-Ohio), chairman of the Senate Permanent Subcommittee on Investigations, opened an inquiry into the government’s system for processing unaccompanied minors. The results are scheduled to be made public Thursday, when Portman plans to chair a hearing on the matter.

“Based on what I have learned to date, I am concerned that the child placement process failures that contributed to the [egg farm] trafficking case are part of a systemic problem rather than a one-off incident,” Portman said in a statement.