Testifying for the first time since President Trump declared a national emergency over illegal immigration, Homeland Security Secretary Kirstjen Nielsen was grilled about administration's family separation policy — at one point telling House Homeland Security chairman Bennie Thompson (D-Miss.): "Sir, we don't use cages for children."Nielsen: "Sir, we don't use cages for children. In the border facilities that you've been to, they were not made to detain children. As the children are processed through, they are in some parts of those facilities. I'm being as clear as I can, sir."Thompson: "Yes or no, are we still putting children in cages?"Nielsen: "To my knowledge, CBP never purposely put a child in a cage."Thompson: "Purposefully or whatever, are we putting children in cages? As of today?Nielsen: "Children are processed at the border facility stations that you've been at --"Thompson: "And I've seen the cages. I just want you to admit that the cages exist."The big picture: Nielsen told the committee that there is a "humanitarian catastrophe" at the southern border with Mexico, and that the U.S. is on pace to encounter close to 1 million people seeking entry this year.She cited recent Customs and Border Protection February apprehension data, which showed U.S. border patrol agents arrested or turned away 76,325 immigrants at the southern border — the highest "encounters" in February in the last 12 years. Of those 76,000, 7,250 were unaccompanied children and 40,385 were people who came with family members.More highlights from Nielsen: "This is a legitimate national emergency. ... We need a barrier. We cannot take operational control with out.""There was no parent who has been deported, to my knowledge, without multiple opportunities to take their children with them.""The consequence of any adult going to jail in this country is they're separated from their child. That wasn't the point of it. The point of it was to increase prosecutions for those breaking the law and not accept any class of aliens."Nielsen said she was unfamiliar with the possible implications of the zero tolerance policy on children's trauma.