Cartels and illegal immigrants have found weak spots along the U.S.- Mexico border. Four feet beneath Nogales, Ariz., agents crawl through a claustrophobic drug-smuggling tunnel, knowing they could come face to face with heavily armed criminals. Trains, with a myriad of hiding places, are also becoming a favorite place for cartels to stash drugs. If the officers aren't careful, they could easily miss a load.
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