Nielsen's comment while testifying before the House Homeland Security Committee came as she defended the Trump administration's policies that resulted in family separations at the southern border.

Nielsen said that parents set to be deported by the U.S. are asked by authorities whether they want to take their U.S. citizen children with them, and again by immigration judges who oversee their cases, many times in conjunction with nongovernmental organizations and the parents' home-country consular officers.

It's unclear how many parents have been deported without their children, something Democrats pressed the Trump administration official about during her testimony Wednesday.

Those separations generally occur when undocumented parents have U.S. citizen children who are not subject to deportation.