The Obama administration failed to protect thousands of Central American children who have flooded across the U.S. border since 2011, leaving them vulnerable to traffickers and to abuses at the hands of government-approved caretakers, a Senate investigation has found.

The Office of Refugee Resettlement, an agency of the Department of Health and Human Services, failed to do proper background checks of adults who claimed the children, allowed sponsors to take custody of multiple unrelated children, and regularly placed children in homes without visiting the locations, according to a 56-page investigative report released Thursday.

And once the children left federally funded shelters, the report said, the agency permitted their adult sponsors to prevent caseworkers from providing them post-release services.

Sen. Rob Portman (R-Ohio) initiated the six-month investigation after several Guatemalan teens were found in a dilapidated trailer park near Marion, Ohio, where they were being held captive by traffickers and forced to work at a local egg farm. The boys were among more than 125,000 unaccompanied minors who have surged into the United States since 2011, fleeing violence and unrest in Guatemala, Honduras and El Salvador.

“It is intolerable that human trafficking — modern-day slavery — could occur in our own backyard,” Portman said in a written statement. “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.”

The report concluded that administration “policies and procedures were inadequate to protect the children in the agency’s care.”

HHS spokesman Mark Weber said in a statement that the agency would “review the committee’s findings carefully and continue to work to ensure the best care for the children we serve.”

The report was released ahead of a hearing Thursday before the Senate Permanent Subcommittee on Investigations, which Portman co-chairs with Sen. Claire McCaskill (D-Mo.). It detailed nearly 30 cases where unaccompanied children had been trafficked after federal officials released them to sponsors or where there were “serious trafficking indicators.”

“HHS places children with individuals about whom it knows relatively little and without verifying the limited information provided by sponsors about their alleged relationship with the child,” the report said.

For example, one Guatemalan boy planned to live with his uncle in Virginia. But when the uncle refused to take the boy, he ended up with another sponsor, who forced him to work nearly 12 hours a day to repay a $6,500 smuggling debt, which the sponsor later increased to $10,900, the report said.

A boy from El Salvador was released to his father even though he told a caseworker that his father had a history of beating him, including hitting him with an electrical cord. In September, the boy alerted authorities that his father was forcing him to work for little or no pay, the report said; a post-release service worker later found the boy was being kept in a basement and given little food.

The Senate investigation began in July after federal prosecutors indicted six people in connection with the Marion labor-trafficking scheme, which involved at least eight minors and two adults from the Huehuetenango region of Guatemala.

One defendant, Aroldo Castillo-Serrano, 33, used associates to file false applications with the government agency tasked with caring for the children, and bring them to Ohio, where he kept them in squalid conditions in a trailer park and forced them to work 12-hour days, at least six days a week, for little pay. Castillo-Serrano has pleaded guilty to labor-trafficking charges and awaits sentencing in the Northern District of Ohio in Toledo.

The FBI raided the trailer park in December 2014, rescuing the boys, but the Senate investigation says federal officials could have discovered the scheme far sooner.

In August 2014, a child-welfare caseworker attempted to visit one of the children, who had been approved for post-release services because of reported mental-health problems, according to the report.

The caseworker went to the address listed for the child, but the person who answered the door said the child didn’t live there, the report added. When the caseworker finally found the child’s sponsor, the sponsor blocked the caseworker from talking to the child.

Instead of investigating further, the caseworker closed the child’s case file, the report said, citing “ORR policy which states that the Post Release Services are voluntary and sponsor refused services.”

That child was found months later, living 50 miles away from the sponsor’s home and working at the egg farm, according to the report. The child’s sponsor was later indicted.