Thousands of migrant children who crossed the southern border into the U.S. have reported they were sexually assaulted while in government custody, according to Department of Health and Human Services documents released Tuesday by Rep. Ted Deutch's office.

In the past four years, 4,556 children said they were sexually assaulted while in the care of Health and Human Services' Office of Refugee Resettlement, which takes custody of unaccompanied minors who cross the southern border alone and those who are separated from their families.

Allegations go back to 2015, meaning the reported assaults started under the Obama administration. But the allegations have increased in the past two years after the Trump administration's "zero tolerance" policy that led to at least 2,800 family separations flooding the department with additional children.

The data show the majority of the alleged assaults were carried out by other minors in custody, but at least 178 were carried out by staff.

"These documents detail an environment of systemic sexual assaults by staff on unaccompanied children," said Deutch, a Democrat from Florida, in a House Judiciary Committee meeting Tuesday. "These documents tell us that there is a problem with adults, employees of HHS, sexually abusing children."

Cdr. Jonathan White, who has overseen the care of migrant children at Health and Human Services, responded angrily to the congressman, saying the government officials at his department have never been accused of such a crime.

"Those are not HHS staff in any of those allegations," White said.

Instead, the department contracts with more than 100 local shelters that house and care for children in its custody. Those facilities are inspected by state child welfare officials, and criminal charges can be filed against employees by state or federal prosecutors.

When a sexual assault is reported to Health and Human Services, White said, it is investigated fully, and those found to be legitimate are referred to the Justice Department of Justice for prosecution.

Data provided by Deutch's office show that of the 4,556 complaints investigated by Health and Human Services, 1,303 – 29 percent – were sent to the Justice Department for further review. White said the vast majority of those cases later proved to be unfounded.

"Any time a child is abused in the care of (the refugee resettlement office) is one time too many," White said Tuesday.

The data were included as part of a large set of documents provided to the Judiciary Committee on the eve of Tuesday's hearing. At the start of the hearing, Chairman Jerry Nadler, D-N.Y., complained about the last-minute document dump, arguing that it took the administration six weeks to answer their questions.

"The Department of Justice and the Department of Homeland Security until last night stonewalled the legitimate request for information by this committee," Nadler said. "That is absolutely inexcusable."

The House Oversight and Reform Committee took it a step further Tuesday, issuing the first subpoenas against the Trump administration after failing to receive answers on separated migrant families that were first posed in July. The subpoenas were directed at Attorney General William Bar, Homeland Security Secretary Kirstjen Nielsen and Health and Human Services Secretary Alex Azar.

"When our own government rips vulnerable children, toddlers and even infants from the arms of their mothers and fathers with no plan to reunite them, that is government-sponsored child abuse," said committee chairman Rep. Elijah Cummings, D-Md. "It is our job to step in and protect those children. Further delay is not an option."

Tuesday's hearing was the second time House Democrats have grilled the administration over family separations. The House Energy and Commerce questioned Health and Human Services officials on Feb. 7 about last summer's family separations and the separations that continue to happen when Homeland Security agents decide migrant parents pose a danger to their children.

U.S. District Judge Dana Sabraw also is considering a request from the American Civil Liberties Union to vastly expand the number of separated migrant families the government must identify and possibly reunite. In June, Sabraw ordered the administration to reunite more than 2,800 families that were separated at the time. But media reports and an internal government watchdog revealed that the administration was systematically separating families a full year before it formally announced its zero-tolerance policy, possibly leading to thousands of additional separations.

The ACLU said the government needs to account for all those families. Sabraw is expected to decide on that request in the coming days.

The Department of Homeland Security also is dealing with a spate of deaths in its custody. Three migrants have died after crossing the border since December, and an Honduran woman delivered a stillborn baby at an immigration detention facility in Texas on Feb. 21.

Families on the border:Despite ban, separating migrant families at the border continues in some cases

Deaths in government custody:Honduran migrant delivers stillborn baby while in ICE custody in Texas