An unpublished Homeland Security Department report obtained by the Associated Press concluded that the Border Patrol caught only 54 percent of those entering illegally from Mexico in fiscal year 2015. DHS previously announced it had an 81 percent success rate for that year.

The leaked report AP obtained was completed in May but not published. Commenting on the report DHS spokeswoman Marsha Catron said "DHS does not believe it is in the public interest to release, and it would be irresponsible to make policy or other judgments on the basis of analysis that is incomplete and remains a work in progress."

The report, prepared by the Institute for Defense Analyses, analyzes illegal border crossings going back to 2000. It estimated that 170,000 escaped capture in FY15, 210,000 in FY14, and 1.7 million in FY05. The numbers exclude people who eluded border crossing officials and or who entered by sea. Adding estimates for such people increases the FY15 number to 200,000, the FY14 number to 260,000, and the FY05 number to 1.9 million.

Before the last two years, DHS had publicly released the number of Border Patrol arrests but not the number who got away. The agency has since released an "interdiction effectiveness rate" that considers all who attempt to enter illegally on the Mexican border. That rate was 81 percent in FY15. It includes those who turned around after being spotted and asylum-seekers, the numbers of which have risen exponentially since the 2014 border surge. There were 170,000 asylum seekers in 2014, but only 20,000 a decade before.

The inclusion of asylum seekers largely explains why DHS’ reported success rate was so much higher. But since the vast majority of asylum seekers voluntarily surrender to Border Patrol Agents -- knowing they will be allowed to remain – it is an open question as to whether the agency’s metric more accurately depicts success.

Read more from the Associated Press.