A driver claims he buys used cars over the border to import into the U.S., but an X-ray reveals something suspicious in his spare tire; when a traveler returning from Mexico declares snacks at Customs, a Fish and Wildlife expert is called in.

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An officer suspects a car is carrying more than innocent Santeria sprays.

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Officers find an 18-wheeler carrying more than a load of takeout containers. A couple raises suspicion after declaring bulldog semen, but the labeling suggests the box may actually contain stallion semen -- a valuable and tightly regulated substace.

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During his first day on the job, a rookie officer relies on his training, sending a hesitant driver for further inspection -- and his hunch pays off. When a traveler claims to be visiting his dentist bimonthly in Mexico, officers investigate further.

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A traveler is caught with blades for cockfighting, a cartel-run blood sport. Every year, Mexico grows millions of dollars of flowers for the U.S., but cartels are taking advantage of the influx of shipments during major holidays.

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Two men arrive in an SUV carrying enough suitcases for an entire family. Officers discover it's no family car -- it's a Kevlar-lined, bulletproof tank, the type that the cartels use to transport high-ranking gang mambers and attack law enforcement.

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Cartels hope CBP won't inspect happy families, but officers know not to fall for that smuggling trick. Due to avian flu, live chickens can't be brought into the U.S.

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Each year, CBP recovers over 1,300 stolen vehicles at ports of entry. Many are heading to the black market, while cartels use others for trafficking. When officers receive a be on the lookout (BOLO) alert for a luxury car, they take it seriously.

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Tourist Day attracts those who move south for the winter. Officers prepare for smugglers hiding inside crowds and parties fueled by alcohol and drugs.

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Officers are caught off guard when a high-end sports car arrives at the border packing heat. At the bus terminal, a K9 alerts to a traveler's luggage. Upon investigation, officers find photographs showing large amounts of cash on her phone.

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Customs and Border Protection forms one of the world's largest law enforcement agencies, drawing officers from many backgrounds. From rookies starting out, to veterans who have seen it all: at America's borders, the mission comes first for CBP officers.

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Officers are caught off guard wen a high-end sports car arrives at the border packing heat. At the bus terminal, a K9 alerts to a traveler's luggage. Upon investigation, officers find photographs showing large amounts of cash on her phone.

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While some officers have years of experience, others are stepping up for the first time -- fresh out of training, and ready to prove themselves. Because at the border, a rookie's first day could be a smugglers's last chance.

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Every year, CBP officers process thousands of vehicles. Some look ordinary, and other are built to deceive -- because at the border, four wheels and an engine could be a cover for contraband.

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Each year, CBP officers inspect thousands of animals, including family pets and exotic wildlife -- because at the border, the threat may come from a different species. Some look innocent, while others carry hidden dangers.

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Millions of travelers cross the U.S. borders every year. And while some are innocent couples, others are partners in crime. Because at the border, love isn't always what it seems.

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Drug cartels are estimated to control up to one-third of Mexico's territory -- there's an insatiable demand for guns and ammo south of the border, and a ready supply of Americans willing to smuggle them. CBP is ready to intercept.

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As they pursue profit, drug cartels are constantly adapting -- hiding deadly narcotics where few would ever expect to find them.

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Mexican cartels earn up to $40 billion annually from trafficking drugs like cocaine, fentanyl and meth into the U.S.

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