A special team of Pima County Sheriff's Department deputies makes daily patrols through a clandestine international trade corridor, which extends from the Mexican border across deserts, over mountains and through rural neighborhoods of the Northwest.
Members of the Border Crime Unit roam the fringes of Pima County, on the hunt for northbound drugs and southbound guns and money. The unit's deputies regularly patrol the Avra Valley area of the Northwest, where contraband and illegal immigrants flow daily en route to stash houses in Tucson and Phoenix.
"They go under the fence, they go over the fence and they go through the fence," said Lt. Jeffrey Palmer, who heads up the Border Crime Unit.
Pima County Sheriff Clarence Dupnik instituted the unit out of the now-defunct Street Crime Unit in 2007, after a group of bandits killed three illegal immigrants in the wilderness near Green Valley.
Working in cooperation with the Department of Homeland Security's Immigration and Customs Enforcement agency, Pima County's Border Crime Unit has been trained and authorized to perform some functions of immigration officers.
About 70 law enforcement agencies across the country have similar authorization under section 287(g) the 1996 Illegal Immigration Reform and Immigrant Responsibility Act and title 19 of U.S. Code. Arizona's recently passed immigration law would similarly deputize all law enforcement agencies across the state.
Today, the Border Crime Unit has 18 deputies, three sergeants and a pair of U.S. Border Patrol agents who report to Palmer. The agents were included to maintain cooperation and communication with Border Patrol.
The unit also has at its disposal helicopters and a pair of small fixed-wing planes outfitted with sensors used to track movement across the border and through the desert.
Many of the illegal immigrants the Border Crime Unit encounters have made the daunting trek on foot through portions of the Tohono O'odham reservation and across mountain ranges to pick-up points at rural crossroads and homes in Avra Valley.
Depending upon their destination, illegal immigrants and smugglers can make almost the entire journey to the outskirts of Phoenix off-road, by following the trails and dirt roads in the wilderness west of Interstate 10.
"They don't even have to touch asphalt until they get to Phoenix," said Deputy Vincent Lopez, as he walks into a small patch of desert shrubs and mesquite trees near Trico Road and an unnamed dirt driveway leading to a sparse cluster of mobile homes and a church.
Discarded backpacks, jackets and water bottles with Spanish labels fill the edges of the small clearing just yards off the roadway. Across Trico, a dirt road follows a stretch of power lines west into the mountains.
In the full sun of a 90-degree day, the water bottles remain half full, a sign they could have been left as recently as the previous night, Lopez said.
Few cars pass by the quiet corner. It's one of many in the area where migrants, in groups of a few people to dozens and more, await transport.
The day before, Border Crime Unit deputies apprehended 25 illegal immigrants and eight backpacks, each filled with 40 to 50 pounds of marijuana.
"The size of the groups amazes me," Lopez said. "We've monitored groups up to 70 people."
Not long ago, deputies arrested a nearby resident who had been harboring illegal immigrants in a mobile home on their property. A beacon atop the home, when lighted, signaled to the crossers that it was safe to come in and wait for their rides, Lopez said.
With illegal immigrant traffic common throughout the region, sheriff's deputies often respond to calls of trespassing.
"This is where 90 percent of our prowler calls come from," Lopez said.
More often, however, Border Crime Unit squads rely on their constant presence in the desert, watching and waiting for the signs aliens and drug smugglers leave behind. The Border Patrol has trained many of the Border Crime Unit deputies in the art and science of tracking.
"The story is here for somebody to read it," Lopez said as he stood over a line of footprints leading in and out of a lonely stretch of desert near a crossroads of Sandario Road.
On this day, Lopez reads the signs into a thicket of mesquite trees and shrubs where piles of torn burlap and empty potato sacks litter the desert floor. The sacks are remnants of marijuana bales, each holding at least 50 pounds, walked across the border from Mexico on the backs of people in the employ of one of that country's many drug cartels.
ATV tracks also fill the area; they are the vehicles likely used to transport the contraband to nearby stash houses. Also from this spot, and scores of others around Southern Arizona, the drug shipments that arrive on people's backs are taken to numerous warehousing sites in the Tucson area for processing and shipment to disparate parts of the country.
It's a multi-billion dollar operation for the cartels in Mexico, and one that they have learned to execute with increased precision.
"They have scouts on every hilltop," Palmer said. "Every time we're out there, we're being watched."
Border Crime Unit squads regularly intercept radio conversations, where the cartel scouts alert smugglers and their armed guards of Border Patrol and deputies' movements.
Other times, Border Crime Unit deputies have seen scouts sent out ahead of drug mules to draw out any law enforcement in the area.
"It's a cat and mouse game," Palmer said.
The cartels also have scouts in the city, people paid to notify the smugglers when Border Patrol helicopters set out from the airport in Tucson, Palmer said.
They have shown an aptitude for creativity and an ability to adapt their methods to keep trade routes open.
"There's a runway out here," Lopez said as he turned his truck into the desert near Anway and Avra Valley roads.
A short way off the road, hidden from sight by the sparse vegetation, lays a series of clearings. The area looks to be a long-abandoned cotton field, but its long, flat stretches of hard-pack desert floor suffice as a landing strip for small aircraft or ultralight planes, used by smugglers to deliver contraband shipments.
"There's a neighborhood less than 500 yards away," Lopez said, looking across Avra Valley Road. "Those people probably don't even realize what happens here."
Lopez said an ultralight easily could bring as much as a few hundred pounds of drugs to a makeshift runway like this. Once they touch down, they meet an accomplice who takes the contraband and refuels the aircraft, which follow landmarks like the Brawley Wash or power lines that lead to Mexico.
U.S. Rep. Gabrielle Giffords, a Democrat who represents much of southern Arizona, recently introduced legislation that would impose strict penalties on those who smuggle drugs using ultralight and other low-flying aircraft.
In three years, the Border Crime Unit has had a measurable impact.
The unit has run more than 1,000 operations, seized 25,000 pounds of marijuana, confiscated $1.1 million in cash, taken 123 stolen vehicles and appropriated 42 weapons.
In addition, the unit has referred nearly 1,300 suspected illegal immigrants to Border Patrol custody, and made 416 felony and misdemeanor arrests.