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Thousands of allegations that migrant children in U.S. custody were subjected to sexual abuse, harassment or inappropriate sexual conduct were reported over a four-year span to the government agency tasked with overseeing their care, according to documents released Tuesday by a Democratic lawmaker.

The documents, provided by the office of Rep. Ted Deutch of Florida, show more than 4,550 allegations were reported to the Office of Refugee Resettlement between fiscal years 2015 and 2018. That agency, which prepared the documents, oversees the care of unaccompanied and separated migrant children and is a part of the Department of Health and Human Services.

During the same period, about 1,300 sexual abuse allegations were reported to the Department of Justice, according to the documents. Nearly 180 involved allegations of staff committing sexual abuse against migrant children at government-run shelters. The documents were first reported by Axios.

Deutch said Tuesday during a House hearing on migrant family separations that he was “deeply concerned” about the “high number” of allegations of sexual assaults on migrant children in government custody.

“These documents detail an environment of systemic sexual assaults by staff on unaccompanied children,” he said.

He added the records show there have been 154 allegations of sexual assault by staff on unaccompanied migrant children in facilities under U.S. custody in the past three years, appearing to refer to the numbers from fiscal years 2016 to 2018.

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Deutch said that amounted to an average of “one sexual assault by HHS staff on an unaccompanied minor per week.” He then asked if concerns about migrant children being potentially exposed to sexual abuse factored into discussions about migrant family separations.

The comment drew an intense response from Cmdr. Jonathan White, a career staffer with Health and Human Services who led the department’s efforts to reunite migrant children with their families after the Trump administration’s “zero tolerance” policy.

“Let me first correct an error, those are not HHS staff in any of those allegations. That statement is false,” White said.

The government contracts nonprofit and private companies to operate shelters where migrant children are held before they are released to a sponsor, usually a parent or close relative already in the country.

Jonathan Hayes, the acting director of the Office of Refugee Resettlement, said in a statement Tuesday evening to NBC News that Deutch leveled an "unfounded assertion" that federal HHS staff were the subject of abuse allegations.

"His knowing mischaracterization of the data — and his impugning of the ORR federal staff — was an immoral and indecent insult to all of the career civil servants who are dedicated to ensuring the health, safety, and welfare of the children" in the unaccompanied children program," he said. Hayes called on Deutch to apologize for the comments.

Hayes said the safety of children in its care "is our top concern, adding, "Most of our care facilities are licensed by the respective state for child care services, and operate under both state and federal oversight."

The "vast majority" of the thousands of claims reported to his agency involved child on child allegations of "inappropriate sexual behavior," he said. The most serious allegations, those labeled as sexual abuse, are reported to both his office and the Department of Justice, with the "vast majority" of those also involving allegations where the accused are other children, not staff, he said. None of those allegations involved federal staff within his agency, he added.

"These allegations were all fully investigated and remedial action was taken where appropriate," he said.

During a contentious back and forth at Tuesday's hearing, Deutch tried to interrupt White, who added, “You are speaking of allegations of sexual abuse against members of my team.”

“I saw thousands of cases of sexual abuse if not by HHS staff then by the people that HHS staff oversees, I will make that clarification. It doesn’t make what happened any less horrific,” Deutch responded.

White said the 154 cases Deutch was referring to were allegations and not confirmed reports of sexual assaults, adding, “This is a longer conversation.”

“In every conversation that we had about separation, we opposed separation,” White added.

White previously told a House subcommittee that the agency tasked with caring for unaccompanied migrant children would never "have supported" family separations at the border.