STORY: Goose the Golden Retriever is sniffing for fentanyl.

Suddenly, he stops and sits - that's the cue.

:: San Ysidro, California

He's trained to sniff out drugs, guns, and money at America's land border crossings, airports, and seaports.

:: Custom and Border Protection

In 2017, the U.S. Customs and Border Protection - or CBP - took the then-unprecedented step of training drug-sniffing dogs to detect fentanyl after seeing a rise in the illicit drug...

...and the epidemic of related overdoses that followed.

An estimated 75,000 people died from synthetic opioid overdoses in 2023, according to the U.S. Centers for Disease Control and Prevention.

Most involved fentanyl.

The canine program has proved crucial to the agency's efforts.

The vast majority of CBP fentanyl seizures occur at legal border crossings in Arizona and California and involve canines, according to CBP data provided to Reuters.

Most convicted fentanyl traffickers in recent years have been U.S. citizens.

:: Custom and Border Protection

At ports like San Ysidro in California, millions of dollars in technology allow CBP to scan vehicles and analyze data, targeting possible smugglers.

But it's here that the K9s' noses shine, said Pete Flores, the acting deputy commissioner for CBP.

:: Pete Flores, Acting Deputy Commissioner, U.S. Customs and Border Protection

"A port like San Ysidro, you have 34 lanes worth of traffic. You have vehicles seven, eight, 10 deep. So in a space of a matter of minutes, you have 400-500 vehicles in this space moving in seconds. So how do we then ensure that we're putting our best foot forward in addressing threats and mitigating threats that we're seeing coming across our ports of entry? Our assets and what we have from a canine perspective is one of the better tools that we have to do that."

Dogs have a sense of smell that is exponentially more powerful than humans.

They have up to 200 times more olfactory receptors, according to a 2022 study published in the peer-reviewed Journal of Neuroscience.

That means they can rapidly sweep through vehicle traffic, search suspicious cars and check lines of passengers.

They are particularly useful for uncovering fentanyl, which can be moved in small quantities as pills or powder.

:: Front Royal, Virginia

At the CBP Field Operations Canine Academy in Front Royal, Virginia, this pooch is learning to sniff out the drug in an airport setting.

When customs officers from across the country arrive, they're paired with their new four-legged partners.

It's part of a four-to-six-month process to teach the dogs how to seek out the contraband.

Academy director Donna Sifford said it takes three days on average to learn the new smell.

:: Donna Sifford, Director, Field Operations Canine Academy, U.S. Customs and Border Protection

"We put them inside this tube, and we actually take it, and we throw it out in the middle of the field, and the dog runs, and he grabs it, and he starts running around. And as he's running around playing with his toy, he's breathing in all these odors. And so he learns very quickly through repetition that, "If I find this, it means this odor."

Before CBP could start training on fentanyl, the agency needed to develop safety protocols.

Trainers always carry four doses of the opioid overdose drug Naloxone - which can also be administered to dogs.

Sifford said it hasn't been needed so far.

Still, the dogs have limitations.

They can typically only be out searching vehicles or people for about 20 minutes before they need a break, officials said.

"If a little warmer day such as this, maybe we will take our dogs out a little less."

Sidney Aki is the CBP Director of Field Operations for the San Diego field office.

:: Sidney Aki, Director of Field Operations San Diego, U.S. Customs and Border Protection

"I give them time to take a break, and then another canine will come out."

Drug-sniffing canines can also send false alarms, with studies showing a range of effectiveness.

And while CBP's fentanyl seizures increased in recent years, the agency only appears to intercept a small percentage coming into the U.S.

Despite that, Aki said they're still valuable to the mission.

"The canine program is a tool, one of our tools in our huge repertoire of technology and tools that we utilize to focus on our border security mission. Of course, if we had more canines, we had more personnel partnered up with canines. We would continue to do more and more and more. But again, we would like to look at ourselves as a good balance law enforcement entity. So we are utilizing the best of all that we have."