WASHINGTON — Thousands more immigrant children were separated from their parents under the Trump administration than previously reported and whether they have been reunified is unknown, according to a report released Thursday by the inspector general for the Department of Health and Human Services.

The report found a spike in immigrant family separations beginning in the summer of 2017, a year prior to the "zero tolerance" policy that prosecuted immigrant parents who crossed the border illegally while holding their children separately in HHS custody. The families separated under zero tolerance were represented in a class action lawsuit, where a federal judge ordered that the government reunify them.

However, the government had no such order to reunify children separated prior to "zero tolerance." Some may have been released to family or nonrelative sponsors, but it is not known how many have been reunified.

HHS officials did not keep track of whether children they were releasing from their custody had been separated from their parents at the border or whether they crossed the border without a parent.

"We don't have any information on those children who were released prior to the court order," an official from the HHS Office of Inspector General told reporters on a call Thursday.

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The officials said they based their estimate of "thousands" of separated children on interviews with HHS staff, but they were not able to provide a more specific number.

"Thousands of children may have been separated during an influx that began in 2017, before the accounting required by the Court, and HHS has faced challenges in identifying separated children," the report said.

The Department of Homeland Security disputed the "thousands" reported by the HHS Inspector General, claiming the inspector general did not have evidence to back up the claim. According to DHS statistics, in fiscal year 2017, the border patrol separated 1,065, 46 due to fraud and 1,109 due to medical or security concerns.

A spokeswoman for HHS said the agency "face challenges in identifying separated children."

Because DHS does not routinely tell HHS that the children they send to their care have been separated, HHS workers are now "using interviews of children during the admissions and case assessment process to look for any indicators of potential separation," the spokeswoman said.

Prior to "zero tolerance," children were separated from parents if they had a criminal history, but it is not known whether the criminality was violent, the HHS inspector general officials said. The vast majority of immigrants prosecuted at the border are arrested on charges stemming from illegal re-entry, not violent crimes, according to data compiled by Syracuse University.

DHS did not provide HHS with information about the criminal history of the parents, the officials said, though HHS sought those details.

As NBC News previously reported, the government ran a pilot program for separating migrant families in El Paso, Texas, before they formally announced the policy. Trump ended the "zero tolerance" policy with an executive order on June 20, 2018.

Katie Waldman, a spokeswoman for DHS, said the report "vindicates what DHS has long been saying: for more than a decade it was and continues to be standard for apprehended minors to be separated when the adult is not the parent or legal guardian, the child's safety is at risk or serious criminal activity by the adult."

Waldman said the children separated prior to zero tolerance were not taken simply because the parent had entered the U.S. illegally. She attributed the rise in separations to the increase in families crossing the border overall.

Lee Gelernt, the lead attorney representing families separated under zero tolerance and deputy director of the ACLU’s Immigrants’ Rights Project, said, "This policy was a cruel disaster from the start. This report reaffirms that the government never had a clear picture of how many children it ripped from their parents. We will be back in court over this latest revelation."