Tech vs. smugglers face off at busiest U.S. border crossing (pictures)

SAN YSIDRO, Calif.--It's probably the busiest border crossing in the world, and every day, mixed in with the thousands of law-abiding people coming into the U.S. here from Mexico, smugglers attempt to outfox the law and sneak in all kinds of contraband -- like 38 pounds of marijuana hidden in wrapped packages found inside the gas tank of a car on Monday.

From pot to meth, cocaine to hidden illegal immigrants, the list of things that are smuggled in is long. And that's before you even start talking about stopping terrorists from getting into the country.

For U.S. Customs and Border Protection (CBP) officers, stopping the illegal activity is a never-ending battle, even as they attempt to make passage as fast as possible for everyone else.

To help them do their jobs, they have a slew of different technologies at their disposal -- and some very low-tech tools as well. As part of CNET Road Trip 2012, reporter Daniel Terdiman stopped in at San Ysidro to see what CBP is throwing at the problem.

Although much of the illegal activity at the San Ysidro port of entry involves drug smuggling, Customs and Border Protection is also on the lookout for anyone bringing radiological weapons -- or illegal material -- into the country. These yellow portals, which all vehicles crossing into the U.S. here must pass through, quickly monitor for any such material, alerting officers if any is detected.

However, there are all kinds of legal medical materials that might trigger the system -- such as residual chemicals from chemotherapy or stress tests. So if a vehicle sets off an alarm, officers will conduct a secondary search to quickly try to determine what triggered it.

On Monday, after a Customs and Border Protection officer noticed a driver behaving nervously, and a sniffing dog reacted to something in the man's car, officers conducted a search of the vehicle. Using imaging technology, they determined that something in the gas tank wasn't right. They found a panel giving access to the gas tank, and inside turned up 38.44 pounds of marijuana, seen here.

The gas tank access panel was hidden under the cushion in the car's back seat and is easy to see by lifting the cushion.

If officers believe it's warranted, they will drive cars through a set of backscatter panels at secondary inspection that generate imagery of the sides, the top, and the underside of the vehicle, highlighting any anomalies.

This marker on the ground a few hundred feet south of the primary inspection booths designates the actual boundary line between the U.S. and Mexico.

Although most rigorous searches of vehicles takes place at secondary inspection, Customs and Border Patrol officers will sometimes decide they need to conduct a search immediately, as in this photograph, taken before the car being searched had even reached primary inspection.

This is a "buster," a small hand-held device carried by some CBP officers that allows them to inspect small areas of a vehicle for any kind of unexpected material thickness. Like a stud-finder, the device can help the officers locate things like thick metal or organic material that shouldn't be there.

A CBP officer uses her Buster to see if anything is hidden in the tires of this car, which is waiting in line for primary inspection.

This image was taken at a U.S. Border Patrol checkpoint in Arizona using a backscatter machine. U.S. Customs and Border Protection uses similar technology to get imagery of vehicles at its ports of entry, including the one at San Ysidro.

This camera, which is installed in the pre-primary area, recognizes cars' license plate numbers so that by the time a vehicle reaches an officer in a booth, that officer already has received information from law enforcement databases about the vehicle.

A primary inspection booth, where a CBP officer sits and processes people coming through the port of entry from Mexico into the United States. The officer will get information about each vehicle before it pulls up, and often has information about the driver -- or passenger(s) -- as well, allowing him or her to process each vehicle faster.

This device, which is installed in each primary inspection booth, is called an annunciator. It provides an alert when a radiological monitor mounted in the pre-inspection area of each lane detects something on an incoming vehicle.

CBP officers regularly walk around the pre-primary lanes with dogs that are trained to detect anything from currency and firearms to drugs and concealed people.

This is an RFID reader that can automatically read several different kind of official identity documents, including U.S. Passport cards (but not Passport books); Border-crossing cards that allow non-citizens to travel up to 25 miles into the U.S. for up to 30 days; and SENTRI cards, which expedite passage for regular border crossers who have voluntarily gone through a background check and interview.

This helps speed up passage for everyone because the card reader gives the primary inspection officer information on the person using the card before they get to the booth.

A large sign welcomes arrivals to the United States.

Incoming vehicles are segmented into different lanes depending on the category of traveler. Those carrying SENTRI cards can choose from several lanes -- which move much faster than normal lanes. But officers can use these gates to redirect traffic depending on how many people in each category are coming through.

By closing this gate, officers are directing incoming cars to use the next lane over. The decision to close the gate was likely made because of a determination that the flow of traffic had changed.

A sign welcomes new arrivals to the secondary inspection area.

Though most people come through the port of entry in a vehicle, many walk in on foot. Here, a CBP officer asks questions of a woman who has tried to come through without a passport.

These signs alert arrivals to warnings about illegal smuggling and the consequences of breaking the law.