Dozens of women, men and their children, many fleeing poverty and violence in Honduras, Guatamala and El Salvador, arrive at a bus station following release from Customs and Border Protection on June 23, 2018 in McAllen, Texas.

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

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 and whether those children were reunified is not known. Some may have been released to family or non-relative sponsors.

HHS 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.

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.

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.

The Department of Homeland Security 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.