Drug smugglers in Nogales, Sonora are using the border city's sewer and drainage tunnels to circumvent barriers set up by US Customs and Border Protection.

Part of the problem, according to border officials, is geography. Nogales, Sonora sits at a higher elevation than its sister city, Nogales, Arizona. That means that the drainage tunnels in Mexico flow into the United States, creating a challenge for Border Patrol agents.

"Drainage was good for them, because they would only have to dig a short distance across the border and they already have existing drainage so they don't have to dig anymore of a tunnel," said Kevin Hecht, who is the deputy patrol agent in charge of the Nogales Border Patrol office.

[PHOTOS: Inside the cartel drug tunnels]

Hecht says authorities have discovered roughly 110 drug tunnels in the Nogales area since 1990. Nowadays, he says the most common tunnels are the so-called "building to building" tunnels, where the smugglers dig standalone tunnels that cross the border. But the underground infrastructure is still alluring to them, because of the easy access it offers to the border.

CBS 5 Investigates traveled into Nogales, Sonora and videotaped the main river that runs beneath the city. It connects to sewer lines and storm drains and there is evidence that smugglers still use the tunnels as a way to get close to the border. From there, they either try to cut through the barriers and grates put in place by border agents, or dig around them.

"It's not like it used to be. Nowhere near that," said Hecht. But when asked if the threat was gone, he said, "I don't know if it's passed. I think as long as we keep up the proactive approach, we will maintain that."

Copyright 2016 KPHO (KPHO Broadcasting Corporation). All rights reserved.

Copyright 2018 KPHO/KTVK (KPHO Broadcasting Corporation). All rights reserved.


CBS 5 Investigative Reporter

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