The Arizona Cattle Growers' Association is offering a $15,000 reward for information leading to the arrest of the person who fatally shot a prominent southeast Arizona rancher.

The body of 58-year-old Robert Krentz was located before midnight Saturday on his 35,000-acre ranch about 35 miles northeast of Douglas. He was out checking water line and fencing on the land Krentz's family has ranched since 1907.

Cochise County authorities believe Krentz was killed by an illegal immigrant who then fled to Mexico. Foot tracks were identified and followed approximately 20 miles south to the Mexico border by county, state and federal authorities.

In addition to announcing the reward money Tuesday, Cattle Growers' Association officials say a memorial fund has been set up at Wells Fargo Bank to help Krentz's family.

Reached by phone Tuesday at his family's ranch, Andy Krentz, Krentz's oldest son, said his father was a churchgoing man who routinely went out of his way to help those in need.

"My father was a very good family man," Krentz told FoxNews.com. "He supported his kids, supported his family. He went out of his way to help anybody we could without regarding to who they were.  It didn't matter who they were."

Sue Krentz, Krentz's wife, said she was "pretty overwhelmed" by her husband's death, which coincided with her parents' deteriorating health.

"This is icing on the cake," Krentz said.

In 1999, Krentz and his wife told PBS' Religion & Ethics Newsweekly that the Krentz ranch had been broken into and burglarized of $700 worth of items. Despite the crime, Krentz said he helped any illegal immigrant he could.

"And you know, if they come in and ask for water, I'll still give them water," he said. "I -- you know, that's just my nature."

Locally, Krentz was known as a good Samaritan who often helped injured illegal immigrants who tried to cross the desert in an area where summer temperatures often hit 120 degrees.

Krentz radioed his brother Phil between 10:00 and 10:30 a.m. Saturday morning that he had encountered an illegal immigrant, and was supposed to meet Phil on the ranch at noon, according to sheriff's deputies. When Krentz didn't show up, the brother called police, and a search team was soon scouring the sprawling desert 15 miles north of the border, near Douglas, Ariz.

At around 11 p.m., a state police helicopter found Krentz slumped over his ATV, the engine and lights still on. Nearby, his dog lay critically wounded, also hit by a bullet. The dog was put down Sunday morning and will be cremated, its ashes spread on the property along with Krentz's.

Tracker dogs have now followed the tracks of the killer back into Mexico, some 15 miles south.

Police have several scenarios of what might have occurred.

One, a drug cartel scout. The Chirachua mountains in southeast Arizona are 11,000 feet tall, rugged and remote. It is a popular drug corridor and the killer may have been clearing the way for a load of drugs moving north when Krentz surprised him.

Two, the suspect belonged to a band of thieves terrorizing the remote ranches spattered around the area — an idea supported by other ranchers.

"Two days earlier a 9mm and a 9mm Glock had been stolen from a home in Portal," said rancher Roger Barnett. "There is no way to know for sure it was the murder weapon. But the bullet the killed Rob and his dog was a 9mm."

Three, retaliation. The day before the killing, Krentz's brother Phil stopped a caravan of illegal immigrants carrying 280 pounds of marijuana. All eight were arrested by the border patrol and the pot was impounded.

The odd part, according to local land owners, is that the killer was apparently alone. Illegal immigrants crossing the border usually hire coyotes to help them pass and travel in groups or 5, 10, 20 or more.

"I think Rob came to help this guy, and the moment he put that telephone to his ear, he was shot," said rancher Richard Humphries, who lives in nearby Elfrida. "It's happened to me. They don't want you to call the border patrol."

Krentz had both a rifle and a revolver, neither of which he used. And both were still with the ATV when his body was found.

Unless the Mexican government finds the killer, Humphrey doubts they'll ever discover the killer's motivation.

The bigger question now looming is how -- if at all -- the rancher's death will affect the immigration debate.

While Arizona Gov. Jan Brewer and other politicians will call once again for the deployment of the National Guard along the border, few here expect the troops to actually carry guns and provide a deterrent effect.

"What would it show? It would show we're serious about controlling our border, which is not the message this administration wants to send," Humphries lamented.

Advocates of reform will argue the killing is an aberration and that most illegal immigrants come to work, not commit crime.

And while the ranchers here dispute that, as they see illegal immigrants crossing their land everyday, most in Cochise County expect little more than lip service from Washington.

Rancher Roger Barnett was sued for $32 million for pointing his gun at a group of illegal immigrants. He says that immigration advocates in Washington may argue the border is secure, but that Krentz's death puts paid to those claims.

"Obama and [Secretary of Homeland Security Janet] Napolitano are dead wrong about our border being secure," said Barnett.

The Associated Press contributed to this report.