More than 450 parents accused of illegally entering the country may have been deported while their children remain in the U.S., a major stumbling in the federal effort to reunite all the families by Thursday's court-ordered deadline.

The Trump administration, in federal court papers filed Monday, said the cases of 463 parents are "under review." The filing also said 879 parents had been reunited with their children and another 538 have been cleared for reunification.