CAMPO, Calif. (AP) — Richard Gordon is one of the Border Patrol’s best at spotting the smallest human traces in pursuit of people who enter the U.S. illegally from Mexico: dusty footprints, torn cobwebs, broken twigs, overturned pebbles.

It’s a skill he has sharpened over the last 16 years in the craggy, shrub-covered mountains east of San Diego and one that is taking on new importance as gauging border security has emerged as a potential stumbling block to an overhaul of the U.S. immigration system.