Migrant children in the government's care were placed in U.S. homes and left vulnerable to human trafficking due to sometimes non-existent screening by the Department of Health and Human Services, according to a congressional report released Thursday.

The investigation says HHS failed to run background checks on the adults in the sponsors' households, failed to visit sponsors' homes and failed to realize some sponsors were accumulating multiple unrelated children, which can be a sign of human trafficking.

Lawmakers contend the government weakened its child protection policies as it was overwhelmed by tens of thousands of children crossing the border. A Senate subcommittee held a hearing Thursday to release the report and examine weaknesses in the department's placement of migrant children.

At the hearing, HHS officials declined to fully answer many of the senators' questions, at times saying they did not have the legal authority from Congress to follow up on the children.

The investigation by the panel echoes the findings of an Associated Press investigation that found more than two dozen unaccompanied children were sent to homes across the country where they were sexually assaulted, starved or forced to work for little or no pay.

Ohio Sen. Rob Portman, the chairman of the panel, says the HHS placement program for migrant children suffers from "serious, systemic defects."

The congressional investigation and hearing are in response to a case in Portman's home state of Ohio, where six Guatemalan unaccompanied minors were placed with human traffickers, including sponsors and their associates. Lured to the U.S. with the promise of an education, the teens instead were forced to work up to 12 hours a day on egg farms under threats of death.

The report says the department did not conduct any home visits in the Ohio case and performed visits in less than 5 percent of cases overall from 2013 to 2015.

Mark Greenberg, acting assistant secretary for HHS's Administration for Children and Families, testified at the hearing that the Ohio case is a "deeply dismaying event" but said he is not able to discuss details due to an ongoing criminal investigation. He said policies in place at the time were followed.

Lawmakers from both parties bristled at the officials' answers, saying they weren't adequate when the lives of children had been endangered.

The panel's top Democrat, Claire McCaskill of Missouri, said she is "disgusted and angry" by the results of the investigation.

"The bottom line is when a child is admitted into our county, the United States of America should be an example for the world of how we care for those children," McCaskill said.

Sen. Heidi Heitkamp, D-N.D., criticized the overly legal tenor of many of the officials' responses and asked if they understood why the senators were angry. Sen. John McCain, R-Ariz., stopped his line of questioning and left the hearing after saying that the witnesses were "the definition of noncooperative."

Portman said federal officials don't know how many migrant children they've sent to live with convicted criminals across the U.S. over the last three years.

"Perhaps the most troubling, unanswered question is this: how many other cases are there like the Marion trafficking case?" Portman asked. "The answer is HHS doesn't know."

The panel's report says the agency still can't track whether an adult is attempting to sponsor multiple children at the same time. The agency also commonly places unaccompanied children with alleged distant relatives or family friends without setting eyes on the sponsor or their environment. In addition, current policies allow sponsors to prevent children from being contacted by social workers who go to the home for a check-up visit.

The report also notes that HHS did not spend all of the money it had left in the program even though it says it was overwhelmed.

HHS bars releasing children to anyone convicted of child abuse or neglect or violent felonies like homicide and rape. The department says it recently signed a contract to open new shelters, and is strengthening its protection procedures as the number of young migrants is once again rising.

According to emails, agency memos and operations manuals obtained by AP, some under the Freedom of Information Act, the agency relaxed its procedures as the number of young migrants rose in response to spiraling gang and drug violence in Honduras, Guatemala and El Salvador.