PHOENIX — U.S. Customs and Border Protection officials announced Thursday their biggest fentanyl bust ever, saying they captured nearly 254 pounds (114 kilograms) of the deadly synthetic opioid from a secret compartment inside a load of Mexican produce heading into Arizona.

The drug was found hidden Saturday morning in a compartment under the rear floor of a tractor-trailer after a scan during secondary inspection indicated “some anomalies” in the load, and the agency’s police dog team alerted officers to the presence of drugs, Nogales CBP Port Director Michael Humphries said.

Most of the seized fentanyl with an overall street value of about $3.5 million was in white powder form, but about 2 pounds of it (1 kilogram) was contained in pills. Agents also seized nearly 395 pounds (179 kilograms) of methamphetamine with a street value of $1.18 million, Humphries said.

“It is said that a quarter-milligram, or the size of a few grains of salt, of fentanyl, which is a dangerous opioid, can kill a person very quickly,” Humphries said. The seizure, he said, had prevented an immeasurable number of doses of the drug “that could have harmed so many families.”

Mexican traffickers have been increasingly smuggling the drug into the United States, mostly hidden in passenger vehicles and tractor-trailers trying to head through ports of entry in the Nogales, Arizona, and San Diego areas.