Security officers keep watch over a tent encampment housing immigrant children just north of the Mexican border in Tornillo, Texas, June 20, 2018. REUTERS/Mike
The absence of the 463 parents, which U.S. government lawyers said was “under review,” could impede government efforts to reunite separated families by Thursday, the deadline ordered by a federal judge. The filing did not say why the 463 parents had left the country, but government officials previously acknowledged that some parents had been deported without their children.

As of Monday, 879 parents had been reunited with their children, according to the filing.

About 2,500 children were separated from their parents after the Trump administration announced a “zero tolerance” policy in April aimed at discouraging illegal immigration. The policy was ended in June amid an international outcry about the government’s treatment of immigrant children.

U.S. District Judge Dana Sabraw in San Diego ordered last month that the government had to reunite the children with their parents in a case brought by the ACLU.

On Monday, the government also said 917 parents were either not eligible to be reunited or not yet known to be eligible to be reunited with their child. That number includes parents no longer in the country as well as those deemed unsuitable because of criminal convictions or for other reasons.

Immigration advocates have expressed alarm about parents deported without their children, saying it can create problems with the children’s immigration cases.