BBC Autos' Jim Benning spends a day with US Customs and Border Protection agents at the world's busiest border crossing, California’s San Ysidro Port of Entry.

Robert Hood of US Customs and Border Protection stood on the US-Mexico border facing 25 lanes of idling cars that were waiting to enter the United States. Any one of the drivers, he knew well, could be smuggling cocaine, heroin, methamphetamine or undocumented migrants.

Hood’s eyes darted left and right. He glanced at a silver Hyundai, then at a blue Honda.

I tried to follow his eyes, to see what he was seeing. But where I saw only a sea of vehicles and their seemingly bored drivers, Hood spotted something more — something suspicious.

“I see a couple of cars I want to check,” he said.

Located about 15 miles south of San Diego, California, and abutting the Mexican city of Tijuana, the San Ysidro Port of Entry is the busiest land border crossing on Earth. Every day at this checkpoint and another several miles away at Otay Mesa, more than 100,000 people cross between Mexico and the US — people going to work or school, visiting relatives, or embarking on holidays. The volume is staggering.

I had come here on a recent morning to see up-close the massive task facing Customs and Border Protection (CBP) officials. I’d read the wild stories about inventive smugglers who crammed methamphetamine into gas tanks and cocaine into cars’ side panels. I wanted to learn how agents suss out suspicious drivers from the millions of law-abiding citizens who are crossing at the same time.

Of course, this isn’t the kind of place where you can casually drop in. Even before they agreed to show me around, CBP officials conducted a background check to ensure I wasn’t a threat. Then before my visit, a long list of do’s and don’ts arrived aimed at maintaining security and privacy. (Example: “For reasons of officer safety, no names or recognizable photos or video will be allowed of any CBP officer until his/her permission is granted.”)

As I drove south on the interstate, I saw Tijuana spread out before me, the city’s crush of houses and shops pressed up against the border. On the US side, the development is sparser. The small community of San Ysidro sits next to the crossing. Beyond that, to the east and west, are mostly barren hills and ranches — a kind of no man’s land that’s primarily the domain of Border Patrol agents and farmers.

A freeway sign warns, “Last USA Exit.” I pulled off, parked in a border lot and strolled toward the crossing. There was a buzz — at times, uneasy — to the place. Travellers clutching suitcases and duffel bags were changing money and hurrying across the street. Taxis and buses waited. Security was ever-present: Several police officers were handcuffing a man loitering near the sidewalk.

As soon as I arrived at the crossing, I could see why agents were so cautious about visitors. Public affairs officer Angelica De Cima told me that they had just seized 3 kilograms of cocaine, and that further seizures and arrests could ensue. In fact, agents in San Ysidro make an average of six narcotics busts a day.

She led me to an inspection area where a bag of cocaine — US street value: $80,000 — sat atop a cart. Beside the bag was the hollowed-out fire extinguisher where the drug had been stashed in the backseat of a pickup. That morning, before the vehicle even reached an inspection booth, a specially trained dog had sniffed out something suspicious.

“If the canine alerts, the officers will talk to the person, get an interview, and probably remove him in handcuffs”, Hood said. “You don’t want that person to drive the vehicle in. It becomes dangerous for everybody.”

Soon after agents took control of the pickup, an officer tested the fire extinguisher to see if it worked — only days earlier, agents had made another bust involving an extinguisher. “Some of the product came out”, Hood said, “but then it stopped.”

Not surprisingly, border agents see a dizzying array of smuggling attempts as traffickers try to outsmart inspectors. Officers once found 150 pounds of liquid and crystalised methamphetamine in a pickup truck gas tank. They found 162 vials of Ketamine — an anesthetic often used as a recreational drug — tucked in the sunroof of a Lexus. And they once even discovered methamphetamine hidden in cans of cheese and jalapeños.

“I’ve been here 29 years and I think I’ve seen just about everything,” Hood said.

We walked out into the lanes of traffic where drivers were waiting to cross into the US. Cars were lined up as far as the eye could see. Street vendors working on the Tijuana side wandered among the vehicles, selling chips, candy and tchotchkes. Drivers gazed out, tapping steering wheels, counting the minutes.

These lanes represent the first opportunity to identify suspicious vehicles. Agents call the zone “pre-primary” and they employ a technique they call “pulse and surge.”

“We’ll saturate the area with a lot of canines and enforcement and then pull back,” Hood said.

While the dogs are busy sniffing, officers walk among the cars, chatting with drivers and scanning vehicles for anything that appears off.

“You talk to people,” Hood said. “You’re looking for that loose thread in their story. Let’s say the person says he’s going to work. What’s your boss’s name? ‘Ed.’ Okay, you got a phone number for Ed? ‘Well I just started there a week ago.’ They’re prepared only up until a point.”

Officers out in the lanes also tap on gas tanks, listening to the tone of the ping to ensure tanks are loaded only with fuel — and use density meters to determine whether tyres or other spaces are crammed with contraband. These techniques pay off: Roughly 40% of all seizures involving vehicles begin in pre-primary.

The next opportunity to catch smugglers is at the 47 inspection booths, where agents examine passports, ask drivers about the purpose of their journeys and observe behavior. If anything seems amiss, drivers are typically sent to the secondary inspection area, where officers conduct more extensive examinations and use more sophisticated technology, including X-ray scanners and fibre-optic scopes.

“We’re always innovative”, Hood said. “We want new technology. If anybody wants to try anything new for enforcement, they come to us, because if it works here, it’ll work anywhere.”

The approach yields plenty of busts. In fiscal year 2014, agents at ports of entry across California inspected nearly 27m private vehicles and more than a million commercial trucks. In all, they seized more than 76 tonnes of illegal narcotics and arrested more than 33,000 immigration-law violators. While cocaine, heroin and marijuana seizures had dropped from the previous year, methamphetamine busts went up 8%, to nearly 15,000 pounds.

Now the assistant port director, Hood no longer spends his days walking the traffic lanes with canines, but as I stood with him among the cars, I got the impression he sometimes wishes he were.

“I worked out here on a roving team for about five years, and this is the most fun job I’ve ever had,” he said. “It was like hunting or fishing, trying to find the bad guys.”

Public affairs officer De Cima smiled. “They said he was better than the dogs.”

Hood grinned. He credited his success with a combination of intuition, training and experience. “You know these cars and travelers,” he said. “You know the stories. You know the smuggling trends. When something isn’t right, you know it.”

Hood remembered a particularly busy couple of years in which he made more than a hundred narcotics busts. “One month in ’97, I got 43 loads with and without a canine,” he said. “I mostly like to count what I got without the canine.”

Hood recalled once talking to a driver who was pulling a fishing boat. He asked if the guy had caught any fish. “Yeah”, the man said. Hood opened his icebox and spotted a couple of dead fish but no ice. There were no hooks on his fishing poles, either.

Sure enough, Hood said, “He had 2,500 pounds of marijuana in the boat.”

Of course, the agency isn’t perfect. Nobody knows how many smugglers get past agents, or how many migrants successfully sneak into the US.

CBP has more than 44,000 law-enforcement personnel, including Border Patrol agents and CBP officers. That makes CBP bigger than any other law-enforcement agency in the country. Problems are bound to arise, and earlier this year, a law-enforcement task force issued a report critical of CBP, highlighting concerns over corruption and excessive use of force.

De Cima said the agency has already made changes to its use-of-force training and guidelines and is reviewing other recommendations. “US Customs and Border Protection is committed to continually earning the public trust through integrity, accountability and transparency”, she said in a statement.

As we watched agents roam among the cars, I told Hood that I imagined the work must be exhausting — spending hours a day under the hot sun, breathing car exhaust, canvassing the area for smugglers.

Hood looked out at the lines of cars.

“At the end of the day, you’re tired”, he said. Then he flashed a mischievous smile. “But you’re always like, is there one more out here?”

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