The Obama administration sent illegal immigrant children into “modern-day slavery” by turning them over to sponsors who forced them into child labor or subjected them to sexual abuse, members of Congress said Thursday as they demanded that top child protection officials explain how it could have happened.

Social workers don’t verify all sponsors’ identities, don’t make site visits to see the conditions they’re sending the children to, don’t insist on follow-up visits to see how the kids are doing and don’t consider serious criminal records — including child sex charges — automatic disqualification for hosting a child, congressional investigators said.

As a result, the government delivered children into the hands of what amounted to sexual predators or abusers or placed them into abject poverty, investigators detailed in a report about malfeasance at the Department of Health and Human Services’ Office of Refugee Resettlement.

One girl was sent to live with a man who claimed he was her cousin and who had paid to smuggle her into the U.S. It turned out he wasn’t related at all, but instead had paid to bring the girl — with her mother’s encouragement — on the understanding that she would become his wife. She became uncomfortable with their sexual relationship, came forward to report the real story and was taken into child protective services.

In another case, a boy was turned over to a man who posed as a relative, but was in fact connected to smugglers who forced the child to work almost 12 hours a day to pay off the $6,500 his mother gave to smuggle him into the U.S., congressional investigators said. That situation is so prevalent it has earned a name: debt labor.

Worse yet, the administration acknowledged that it can’t account for each of the 90,000 children it processed and released since the surge peaked in 2014.

“It sounds like everything that could go wrong did go wrong,” said Sen. Rob Portman, chairman of the Senate’s Permanent Subcommittee on Investigations, which conducted a six-month investigation into the government’s handling of the tens of thousands of children who have poured across the border in the past few years.

Mark Greenberg, acting assistant secretary at the Administration for Children and Families, the HHS agency that oversees the handling of the children, stumbled for answers during a two-hour grilling, but said his officers were only following their policies.

He insisted that if there was a fault, it lay with Congress, who needed to rewrite the laws if it wanted his social workers to do more to keep children safe.

“What we’re talking about today is our understanding under the law,” he said.

The Obama administration admits it was overwhelmed when unaccompanied children — those sent on the treacherous journey north without a parent or guardian in tow — streamed across the border at the rate of more than 10,000 a month during the peak in the summer of 2014.

Local communities waged “not in my backyard” campaigns to keep the children from being housed at facilities near them, so the administration looked to quickly process and release the kids. Part of that meant relaxing the checks that were performed.

The Washington Times reported in July 2014, at the height of the surge, that advocates predicted children would be sent to unsafe homes, with one group estimating that as many as 10 percent of the children were being sent to live in unacceptable or dangerous conditions.

But 18 months on, the Obama administration has yet to revoke a single sponsor’s custody agreement, with the administration saying once it has placed a child in the hands of a sponsor — either a relative, family friend or someone else — they no longer have control.

If a sponsor refuses to answer questions and shuts the door in the face of a social worker, there’s nothing the administration can do, Mr. Greenberg told the Senate panel.

“Our view that we don’t have continuing custody after we release a child is a long-standing view,” he said. “If this is an area where Congress wants the law to be different, Congress should change the law.”

HHS did not disqualify families even if the sponsor was an illegal immigrant in danger of being deported himself.

Home visits are made in just 4 percent of the tens of thousands of cases, and it wasn’t until earlier this week — years into the unaccompanied minor crisis — that HHS adopted a new policy preventing children from being shipped to homes where someone has been convicted of a sex crime.

“We’re talking about felony convictions for child abuse. Hello?” said a frustrated Sen. Claire McCaskill, Missouri Democrat.

About 90 percent of the children were sent to live with parents or close relatives, but that left thousands who were placed with other sponsors — often people claiming to be family friends.

The subcommittee investigation found some sponsors tried to claim multiple children, and some addresses were repeatedly listed on sponsorship forms, suggesting that government officials should have spotted something wrong.

In the worst public case so far, investigators said human traffickers used the government’s placement program to sneak kids from Guatemala to the U.S., where HHS processed them at the border, then delivered them to supposed family friends. But the friends turned out to be sponsors-for-hire who, as soon as they collected the kids from HHS, turned them over to the traffickers who were running an egg farm in Marion County, Ohio, and needed the children for cheap labor.

The children were forced to work 12-hour days, six or seven days a week, and lived together in a dilapidated trailer. The traffickers withheld paychecks and threatened their families back home in Guatemala to intimidate the children, Mr. Portman said.

“It is intolerable that human trafficking — modern-day slavery — could occur in our own backyard. But what makes the Marion cases even more alarming is that a U.S. government agency was responsible for delivering some of the victims into the hands of their abusers,” he said.