A large tractor trailer filled with cucumbers pulled up to the Nogales border crossing in Arizona in January. The busy port of entry on the US-Mexico border sees more than 1,500 trucks pass through it daily, but something about this specific trailer made local officials suspicious.

Rather than waving the truck through, US Customs and Border Protection officers had a trained dog sniff around it and ran the vehicle through a room-size X-ray machine. What they saw in the grainy black-and-white X-ray images was jaw-dropping.

It turned out to be the largest stash of the synthetic opioid fentanyl ever seized in CBP history. Underneath those cucumbers, in a false floor in the rear of the trailer, was nearly 254 pounds of fentanyl and 395 pounds of methamphetamine -- with a street value of about $4.6 million.

"The size of a few grains of salt of fentanyl, which is a dangerous opioid, can kill a person very quickly," Michael Humphries, CBP's Nogales port director, said during a press conference at the time. "We'll use all our resources to prevent the entry of dangerous narcotics into the United States -- our officers, our technology, our canines and everything we can to throw back at them."

President Donald Trump says a physical border wall is necessary to stop the flow of drugs into the US. But data from CBP paints a different picture, showing that the majority of narcotics make their way into the country through ports of entry. In 2018, for example, 90% of heroin, 88% of cocaine, 87% of methamphetamine and 80% of fentanyl seized by officials was smuggled through legal crossings. At these ports of entry, the government is increasingly relying on X-ray technology to detect such illegal drugs.