Thousands more migrant children may have been separated from their families at the southern border than previously thought, US inspectors have found.
The practice began months before the Trump administration announced its migrant separation policy, according to a new US health department report.
Officials earlier said nearly 3,000 children were taken under the "zero tolerance" policy last summer.
The policy sparked global outcry and Mr Trump withdrew it on 20 June 2018.
Under the "zero tolerance" policy, nearly all migrant adults entering the country illegally were prosecuted while their children were placed in shelters or foster care.
The report, issued on Thursday by the inspector general for the Department of Health and Human Services (HHS), the agency which oversaw the children once they were detained at the border, found a "sharp increase" in the number of children who were separated from a parent or guardian even before the policy was announced in the spring of 2018.
"Thousands of children may have been separated during an influx that began in 2017... and HHS has faced challenges in identifying separated children," the report said.
But the total number of separated migrant children is unknown due to a lack of coordination among government agencies, according to the report.
After the report was released, Democratic Senator Dick Durbin called upon Secretary of Homeland Security Kirstjen Nielsen to "accept responsibility for this outrage and resign".
In June President Trump bowed to public pressure and signed an executive order vowing to "keep families together" in migrant detentions.
A week later a federal judge in San Diego ordered the government to reunite families who were separated at the border in a case stemming from the "zero-tolerance" policy.
But the new report found that 118 children were taken from a parent or guardian between July and November, after the court order.
It also warns that "it is not yet clear whether recent changes to [departmental] systems and processes are sufficient to ensure consistent and accurate data about separated children" going forward.
Under previous US administrations, immigrants caught crossing the border for the first time tended to be issued with court summonses and released.