The silhouettes moved so quickly they blurred out of focus in seconds. Squinting, I could barely make them out from where I sat, in a border patrol truck in Nogales, Arizona, overlooking a stretch of hills in Mexico. The Sonoran desert unfurled in front of us; burnt orange and cracked open.

My guide, a Nogales native and agent with U.S. Customs and Border Protection, scanned the ridge. Cartel scouts, he guessed. The hills serve as their home base. The scouts survey the desert, watching out for border patrols during drug and migrant-smuggling operations. Local reports describe them being equipped with semiautomatic rifles, encrypted radios, cellphones and binoculars. “They’re constantly monitoring our movements in order to get people across without getting detected,” the agent told me. 

Nearby, a surveillance tower pierced the cloud-streaked sky. The tower is one of roughly 48 spread out across the 1,950-mile southwestern border between the United States and Mexico. Stretching up to 160 feet tall, the structures are outfitted with high-resolution infrared and daytime cameras, and radar sensors with a seven-mile range, which transmit video and location data to border patrol agents. 

The towers are part of a web of surveillance that blankets the frontier with Mexico, a decades-old U.S. government effort to fortify the southern border whose origins can be traced to the jungles of Vietnam. In 2020, border patrol apprehended nearly 460,000 people at the southwest border. In 2021, nearly 930,900 have been apprehended, with over 180,000 in May alone.