"Before implementation of the Zero Tolerance Policy, when CBP apprehended an alien family unit attempting to enter the United States illegally, it usually placed the adult in civil immigration proceedings without referring him or her for criminal prosecution. CBP only separated apprehended parents from children in limited circumstances — e.g., if the adult had a criminal history or outstanding warrant, or if CBP could not determine whether the adult was the child’s parent or legal guardian. Accordingly, in most instances, family units either remained together in family detention centers operated by ICE while their civil immigration cases were pending, or they were released into the United States with an order to appear in immigration court at a later date. The Zero Tolerance Policy, however, fundamentally changed DHS’ approach to immigration enforcement. In early May 2018, DHS determined that the policy would cover alien adults arriving illegally in the United States with minor children. Because minor children cannot be held in criminal custody with an adult, alien adults who entered the United States illegally would have to be separated from any accompanying minor children when the adults were referred for criminal prosecution. The children, who DHS then deemed to be unaccompanied alien children, were held in DHS custody until they could be transferred to the U.S. Department of Health and Human Services (HHS) Office of Refugee Resettlement, which is responsible for the long-term custodial care and placement of unaccompanied alien children."