Washington (CNN) The Trump administration does not know how many children were separated from a parent or guardian last year, but it's "thousands" more than the 2,737 it has previously acknowledged, according to a new inspector general report.

"More children over a longer period of time were separated by immigration authorities and were referred to HHS for care than is commonly discussed in the public debate. How many more children were separated is unknown," said Ann Maxwell, Assistant Inspector General for Evaluation and Inspections.

The Department of Health and Human Services and the inspector general were unable to identify how many more children were separated because there were "significant challenges in identifying separated children."

"There was no effort underway to identify those children," Maxwell said, adding that "HHS faced significant challenges."

Officials estimated that "thousands of other children" were separated, received by HHS for care and released prior to the June 26, 2018, court order that required the government to identify and reunify certain separated children, and that children continued to be separated after the court order ordering reunification.

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