(CNN) US Customs and Border Protection made the largest seizure of fentanyl in the agency's history on Saturday at the Nogales port of entry on the US-Mexico border, Port Director Michael Humphries announced Thursday.

Officers uncovered 100 packages of fentanyl weighing nearly 254 pounds and with an estimated value of $3.5 million, Humphries said. The fentanyl was mostly in powder form, with some in pill form.

Fentanyl is a synthetic opioid. Unlike morphine or heroin, which are derived from the opium poppy, synthetics such as oxycodone and the extremely potent fentanyl are manufactured purely from chemicals.

Nearly 650 pounds of illegal drugs were removed from the trailer in total. The drugs, which also included almost 395 pounds of methamphetamine, were concealed within a tractor-trailer transporting produce.

Officers also found 300 packages of methamphetamine, which has an estimated value of $1.18 million. This was the third-largest seizure of meth in Arizona ports of entry.

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