The paw print, judging from the size of it, was left by a large cat just a day or two earlier. Emil McCain kneels over it in the sandy bottom of an Arizona canyon a mile from the U.S.-Mexico border. "This isn't a mountain lion track," McCain says, shaking his head after measuring and then tracing it onto a piece of plexiglass.

The print is huge, four-toed and without claws, like that of a large mountain lion. But the heel pad is too big for a mountain lion, the toes too close to the back pad.

We follow the cat's trail below camel-colored rimrock and live oaks to where it passes an automated camera. For the past year, McCain has operated nearly 30 heat-triggered cameras in these remote mountains that connect the U.S. borderlands to Mexico's northernmost Sierra Madre. When the film is developed days later, McCain's instincts are proved correct. The cat isn't a mountain lion—it's a jaguar, low slung and powerful, moving past yucca and volcanic rock, its eyes reflecting gold in the camera's flash.

For four years, camera traps operated by the Borderlands Jaguar Detection Project, based in Amado, Arizona, have documented two jaguars in these high, arid washes. They may have caught a third animal on film—the cat appears differently patterned than the others. If it is a female, it would be the first one known in the United States in 40 years. It's possible the cats were here all along, unnoticed, or they may be visitors from Mexico. It's also possible that jaguars are returning to—and breeding in—the United States.

The jaguar's range historically extended from northeastern Argentina through Brazil, Central America and Mexico, and followed the mountains along Mexico's Pacific and gulf coasts into Arizona, New Mexico and Texas. But the animals lost ground in the past century. In 1963, a hunter in Arizona's White Mountains shot a female, the last of her sex to be documented in the United States. Two years later, the last legally killed jaguar, a male, was taken by a deer hunter in the Patagonia Mountains, south of Tucson.

In 1969, Arizona outlawed most jaguar hunting, but with no females known to be at large, there was little hope the population could rebound. During the next 25 years, only two jaguars were documented in the United States, both killed: a large male shot in 1971 near the Santa Cruz River by two teenage duck hunters, and another male cornered by hounds in the Dos Cabezas Mountains in 1986.

The animals' prospects brightened in 1996, when Warner Glenn, a rancher and hunting guide from Douglas, Arizona, came across a jaguar in the Peloncillo Mountains of southeastern Arizona. Catching the jaguar on a ledge, Glenn snapped a few pictures, pulled back his hounds and allowed the animal to stride away. Six months later and 150 miles to the west, Tucson houndsmen Jack Childs and Matt Colvin treed a second jaguar near the reservation of the Tohono O’odham Nation. The cat, about 150 pounds and groggy from feeding, allowed himself to be videotaped for an hour.

Not long after Childs' surprise encounter, the hunter became a jaguar researcher, even traveling to Brazil's Pantanal wilderness to study the cats. In 1999, he began placing remote cameras in Arizona where jaguars had been seen in the past. By December 2001, he had his first jaguar photograph: a male weighing between 130 and 150 pounds and later dubbed Macho A. The jaguar looked healthy, well fed and heavily built, with a broad, wide skull that flowed back to a torso shaped like a cylinder of muscle. Macho A turned up on film in August 2003, and again in September 2004. Childs and McCain have since picked up a second male, Macho B, and possibly a third animal.

Experts disagree about what the photographs signify. Alan Rabinowitz of the Wildlife Conservation Society says the animals may merely be dispersing from a dwindling population in Sonora, Mexico, about 130 miles south of Douglas, Arizona. "I think that the [Sonora] population is in serious trouble, and we're almost seeing it act like an organism reaching out and trying its hardest to survive in any way possible." But some of the photographs suggest otherwise. Macho B's canine teeth are yellow and worn, indicating that the cat is 4 to 6 years old, well past the age when he would leave his home turf, McCain says. And if the third camera-trap sighting is of a female jaguar, there's a chance the animals are mating. Craig Miller, a conservationist at Defenders of Wildlife, is hopeful that the U.S. population might recover. "For every one of those jaguars photographed, it could represent two or three more in adjacent habitat," he says.

In March 2003, a Mexico City-based conservation organization called Naturalia purchased a 10,000-acre ranch in Sonora to serve as the core of a private jaguar reserve. Mexican president Vicente Fox proclaimed 2005 the year of the jaguar, and an international convention was held in October on management of the cat.

One rainy day back in the 100-square-mile study area in southeastern Arizona, McCain and I journey to the largest canyon in the mountains. The cameras here have generated 12 photographs of Macho A and Macho B. Two elegant trogons, parrot-like birds whose range is similar to that of the jaguar, call from steep walls. "This site changed the way we think about jaguars in the Southwest," McCain says as he changes the batteries in a camera. "More jaguar photographs have been taken at this spot than in all of the Southwest since the 1950s. This site alone shows these animals are not transients."