U.S. Customs and Border Protection announced Thursday the largest seizure of fentanyl in the agency's history.

Michael Humphreys, Area Port Director for the Nogales Port of Entry, told the press that CBP officers had seized 254 pounds of fentanyl and 395 pounds of methamphetamine at the Nogales port on Saturday morning. The latter amount accounts for 20 percent of all the fentanyl seized in FY 2017 along the entire southwestern border.

According to Humphreys, the drugs were concealed in a tractor-trailer, driven by a 26-year-old Mexican national, that attempted to cross under the pretense of transporting produce. After being alerted by an inspection system, CBP officers investigated the truck and unearthed a hidden section beneath the trailer's floor.

They extracted from it a hundred packages of fentanyl, most in powder form, but ten in pre-pressed pill form, likely to be distributed as counterfeit pharmaceuticals. The fentanyl was collectively worth 3.5 million dollars, according to CBP's estimate. The methamphetamine—in 300 packages—was worth an additional 1.18 million dollars.

The size of the seizure indicates the growing threat of fentanyl smuggling along the southwestern border. Fentanyl names both a class of synthetic opioids and also the most common drug in that group (other instances include drugs like carfentanil and acetylfentanyl). The drug is considered to be 50 to 100 times more potent than morphine.

Other frequently-abused opioids, like morphine or heroin, are usually refined from the sticky, alkaloid-rich latex of the poppy plant. Fentanyl, by contrast, is totally synthesized from simple precursors, meaning it can be efficiently mass-produced in a controlled, industrial setting. Most fentanyl on the illicit market is produced in Chinese factories, before being smuggled into the United States either by mail or over the northern and southern borders.

The size of Saturday's bust indicates how widespread of an issue fentanyl adulteration has become. The drug was not even tracked by CBP four years ago; now it is the leading cause of drug overdose death in the United States, accounting for more than 28,000 deaths in 2017 alone.