DHS did not have the Information Technology (IT) system functionality needed to track separated migrant families during the execution of Zero Tolerance. U.S. Customs and Border Protection (CBP) adopted various ad hoc methods to record and track family separations, but this practice introduced widespread errors. These conditions persisted because CBP did not address known IT deficiencies before the Zero Tolerance Policy was implemented in May 2018. DHS also did not provide adequate guidance to personnel responsible for executing the policy. Because of the IT deficiencies, we could not confirm the total number of families DHS separated during the Zero Tolerance period. DHS estimated Border Patrol agents separated 3,014 children from their families while the policy was in place. DHS also estimated it completed 2,155 reunifications, although this effort continued on for seven months beyond the July 2018 deadline for reunifying children with their parents. However, we conducted a review of DHS data during the Zero Tolerance period and identified 136 children with potential family relationships that were not accurately recorded by CBP. In a broader analysis of DHS data between the dates of October 1, 2017 to February 14, 2019, we identified an additional 1,233 children with potential family relationships not accurately recorded by CBP. Without a reliable accounting of all family relationships, we could not validate the total number of separations, or the completion of reunifications. Although DHS spent thousands of hours and more than $1 million in overtime costs, it did not achieve the original goal of deterring “Catch-and-Release” through the Zero Tolerance Policy. Moreover, the surge in apprehended families during this time period resulted in children being held in CBP facilities beyond the 72-hour legal limit. The Department concurred with all five report recommendations.