The truck was transporting cucumbers, but something much more dangerous was hidden inside: fentanyl, the extremely potent narcotic that has contributed to the epidemic of overdose deaths in the United States in recent years.

The stash, all 254 pounds of it, was seized last Saturday by U.S. Customs and Border Protection officials at the Nogales border crossing in Arizona. Officials announced Thursday it was their largest-ever seizure of fentanyl, and that they also discovered 295 pounds of methamphetamine during the bust.

And it was seized at a legal border crossing.

President Trump has repeated time and again a border wall will prevent illegal drugs from being trafficked into the United States. The Washington Post’s fact-checking team has previously debunked these claims and determined Trump has repeated some version of them at least 71 times.

On a visit to the Drug Enforcement Administration this week, Vice President Pence repeated Trump’s claims, saying the United States needs a border wall to fight drug trafficking.

Vanda Felbab-Brown, a senior fellow at the Brookings Institution who studies drug policy, said most drugs are smuggled into the United States through legal ports of entry. The drugs that do enter the United States are rarely carried by undocumented migrants, she said.

Former cartel members testifying in the trial of drug kingpin Joaquin “El Chapo” Guzman reinforced that concept. Although smugglers have at times used sophisticated tunnel networks to carry drugs into the United States, the majority entered through legal ports of entry, and a wall would not necessarily stop the usage of tunnels. Data from Customs and Border Patrol also states most hard drugs are found during searches at legal border crossings.

“What drug trafficking groups do quite frequently is they send drugs in multiple cars and trucks in multiple ports of entry, and then they tip off U.S. law enforcement that one of the cars has drugs,” Felbab-Brown said. “They sacrifice that loss to generate attention on it and to facilitate the notion that the other cars will not be checked equally as diligently.”

Drugs are not only coming through the southern border. A large amount of drug trafficking is happening at ports in a variety of major U.S. cities, including Boston — far from the border with Mexico.

In his remarks at the DEA this past week, Pence said about 60 percent of marijuana that comes into the United States via the southern border does not pass through legal entry points. But Felbab-Brown said marijuana is “really not the game in town” when it comes to drug smuggling. Marijuana farmers in Mexico have increasingly switched to opium poppy, she said, and with increasing legalization of marijuana in the United States, it is less lucrative for smugglers.

Fentanyl is considered by smugglers to be much easier to transport than marijuana because just a tiny amount can be extremely potent. “Marijuana is very bulky and smelly,” she said. “The amount of high you get out of a gram of marijuana is tiny compared to the amount of high you get out of a gram of fentanyl.”

Officials said the fentanyl seizure this week was so massive it could have provided 100 million lethal doses to users. Its potency is exactly what makes it so dangerous. Border officials packaged it in plastic containers to ensure it did not harm personnel.

A canine team was used to search the truck, where, as The Washington Post’s Nick Miroff reported, “officers opened the false floor compartment to find 400 packages of narcotics — an estimated $3.5 million worth of fentanyl and $1.1 million worth of methamphetamine.”

Sniffing for such a deadly drug is a “tremendous risk for dogs,” said Felbab-Brown. “It’s so potent it can kill dogs by just the smell."

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