Parents deported from the US while their children were left behind is what experts think has happened to hundreds of families.
A court-appointed steering committee began trying to reach the families of more than 1,000 migrant children two years ago.
That's after a federal court ordered the reunification of many families that had been separated at the border under the Trump administration's "zero tolerance" policy in 2018.
According to a court filing Tuesday, the committee has been unable to reach the parents of 545 children.
It believes most of them were deported without their children, and those children are still in the US with a sponsor.
The Justice Department and the American Civil Liberties Union are leading the effort to locate the parents.
The ACLU points out some of the children were babies when they were taken away from their families.