An Arizona police K-9 helped officers seize thousands of fentanyl pills hidden in a car during a traffic stop, authorities said. 

The Cottonwood Police Department said officers were conducting operations on Wednesday when a driver with a cracked windshield was stopped. The dog, named Otto, was used to sniff out any drugs or other illegal substances. 

"A subsequent search of the vehicle revealed a duffel bag in the back of the sedan," the department said in a social media post. "The bag contained five large plastic bags of blue 'M/30' pills."

Upon further investigation, the bag contained 50,000 fentanyl pills that weighed 12 pounds, authorities said. 

DEA WARNS 'RAINBOW FENTANYL' IS A 'DELIBERATE EFFORT BY DRUG TRAFFICKERS TO DRIVE ADDICTION' IN YOUNG PEOPLE

Arizona fentanyl bust

Cottonwood police K-9 Otto poses for a photo in front of a police cruiser and the 50,000 fentanyl pills he helped officer seized during a single traffic stop Wednesday.  (Cottonwood Police Department)

Also found was a "usable amount" of methamphetamine and other items indicating the drugs were being trafficked for sale, they said. 

The driver, 38-year-old Phoenix resident, Carlos Ignacio Urias Meza, was arrested on suspicion of transporting dangerous drugs for sale, possession of drugs and possession of drug paraphernalia, all felonies. 

Authorities are seeing more fentanyl seizures as drug cartel groups continue to expand production at rapid rates, experts say. 

The Sinaloa Cartel and its rival the Jalisco Cartel are responsible for much of the fentanyl manufacturing and smuggling into the U.S., according to former officials and analysts. 

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In 2019, the United States lobbied the Chinese government to bring fentanyl under a stronger regulatory regime. That cut fentanyl shipments from China to North America and opened an opportunity for Mexican cartels to manufacture the drug themselves.

Fox News' Rich Edson and James Levinson contributed to this report.