Nogales, Arizona (CNN) -- Mario Morales keeps his Commando assault rifle propped up on the seat of his patrol vehicle. The car snakes along a dirt road about a half-mile from the Mexican border.
He's always kept the rifle within arm's reach. But in recent weeks, staying armed at all times has taken on a new urgency: Mexican cartels have issued death threats against the police on the Nogales force.
His car rolls to a stop. Morales steps out and points to a nearby hillside where a rusty fence cuts through the desert landscape, separating the United States from Mexico.
"I would not doubt it right now -- we're being watched," Morales says.
At 51, Morales is a member of the Nogales Police Department's SWAT team. He's patrolled the community since he was 20. Over the past three decades, he's seen this region in southeastern Arizona become a drug corridor for Mexican cartels.
Drug runners speed through the canyons, using tree lines to sneak their loot in and out of Mexico. "They load up and load out, and then they're gone," he says.
He has relatives across the border, but he hasn't been to family gatherings in Mexico for three years. It's too risky -- an American police officer fetches a high price for cartel kidnappings.
"I've always tried to be careful," he says. "I never underestimate the cartels in Mexico. I take it very serious. I keep a low profile."
The cartel hit men are ruthless, and he says some were trained by U.S. Special Forces to help Mexico fight the drug war, until they went to the other side. "That is what we're up against."
The recent death threats -- the first ever by Mexican cartels on the police here -- came after two recent busts by off-duty officers. In the first case, two officers were riding horseback when they intercepted a van full of marijuana.
A few days later, the same officers were roping cattle with a crowd of cowboys when they spotted a vehicle with bundles of pot being tossed into the trunk and backseat. The off-duty officers detained two men and called in support. "That was gutsy," says Morales.
The busts netted about $600,000 in marijuana -- relatively small potatoes in the underworld of drug trafficking.
Yet the cartels responded swiftly with a message for Nogales police: When you're not in uniform, you better look the other way or you'll be targeted. The message was relayed to police through an informant.
"They named the Nogales police department officers that were in that area off duty," says police chief Jeffrey Kirkham. "They are not happy, and they are desperate to get that across."
"We're not going to be intimidated," adds Kirkham, the police chief for the last six months. "We're going to continue with our operations. In fact, we're going to step up our operations."
Nogales, population 24,000, is the largest border town in this region. Its downtown is a vibrant community with a Latino feel. Mexican and American flags hang outside storefronts. A quaint plaza is filled with children and parents alike. Outside of town, homes are built just a few feet from the fence dividing the two nations. Mexican neighbors live directly on the other side.
And while the region is a major drug corridor, Nogales has an extremely low crime rate. There has only been one murder in the past three years. By contrast, the police chief says, just across the border there have already been 126 drug-related murders this year.
Police say the cartels are being squeezed at the border and the drug lords are angry their profits are being cut into. More than $10 million in drugs have been confiscated in Santa Cruz County this year. And last year, the town of Nogales captured headlines when U.S. Border Patrol agents found a sophisticated drug-smuggling tunnel that went under the border fence.
The 60-member Nogales police force has now been told to keep weapons on them at all times. They're even encouraged to wear body armor when off duty. The officers remain in constant communication so their whereabouts are known.
"This is the first time Nogales police officers have ever been threatened by anyone in drug trafficking," says chief Kirkham. "I take a death threat any time against a police officer -- locally, federally or state -- as very serious."
At one point, while Morales is speaking with CNN, he turns down his radio for a few minutes. The department issues an all-points bulletin for him.
"It's very dangerous. You have to be very careful," he says.
The lifelong resident of Nogales then climbs back into his police car. Dust kicks up along the barren road. The patrols never stop.
CNN's Traci Tamura contributed to this report.