DOUGLAS, Ariz. -- A deadly spasm of drug-provoked violence in northern Mexico last month has some border residents worried that it is only a matter of time before it spills over the border into Arizona.

Cochise County Sheriff Larry Dever said law enforcement officials share a common belief that the violence spawned between drug cartels operating south of the border inevitably will end up here.

"These criminal syndicates know no borders and use the border to their advantage as a curtain or veil to cover their activities. But in no way is it a barrier to them."

Ranchers and merchants in the border towns say they've seen the fallout from the violence and worry about more to come, either in Mexico or in Arizona itself.

Some 50 drug hit men convoyed into Cananea in northern Sonora state on May 16 and killed seven people, including five police officers whom officials believe were targeted for betraying an agreement with a drug cartel. Army troops and police then pursued the gunmen, identified as "Zetas," former Mexican army elite soldiers, through rugged mountains and, according to Mexican authorities, killed 16.

The incident followed a spate of shootings in southern Arizona in which gunmen trying to interdict drug loads killed illegal immigrants in the vehicles they had attempted to stop.

The Cananea shootings were felt quickly in the towns and remote ranches straddling the Arizona-Mexico border.

Some 35 miles west of Douglas, near the twin border cities of Naco, a hundred or more illegal immigrants daily cut across the San Jose Ranch owned by Jack Ladd and his son, John. The men said the violence around Cananea put a quick stop to the immigrant foot traffic on the ranch, which stretches along more than 10 miles of the Mexican border.

Jack Ladd said migrants apparently stayed holed up in staging areas in and around Naco for about a week, and John Ladd said he is convinced drug cartel-associated criminal activity has been ongoing in the area since July and now has engulfed migrants.

"They're running the people (illegal immigrants) now, too," he said. "Instead of the mom-and-pop taxi service out of Naco, Sonora, it's the cartel that's doing it," he said.

"They're associated with the drugs and the people, and it's big business. The thievery has escalated; they're stealing everything that isn't bolted down on the ranch."

Ladd said a Mexican rancher friend told him that cartel operators were behind the thefts and warned, " 'Don't mess with them.' " He predicted a major incident along the border was just a matter of time.

Others say Arizonans shouldn't worry that the violence will move north.

"It's ridiculous to think that there would be any spillover," said Douglas Mayor Ray Borane. "There's no necessity for it."

As he sees it, the only way drug enforcers would venture into Arizona to take out a target would be if they were pursuing rival drug operatives or if they thought a hit would be easier on the American side.

Douglas police chief Charles Austin said he has no intelligence to show that there is an immediate risk of the violence spreading, although there is that chance.

"It would be foolish not to acknowledge that it's a remote possibility," Austin said. "At a time like this you remain sensitive to the added possibilities, but we've not gone to any kind of added staffing."

Several Douglas businessmen and merchants say there's no doubt that the shootings have adversely affected commerce, which is heavily reliant upon consumers from Mexico. And there's genuine concern that the violence will become an integral part of life in Agua Prieta, Mexico, Douglas' sister city -- which many locals consider part of the same community.

"They're worried about the violence becoming endemic in Agua Prieta," said Douglas native Gerry Bohmfalk, who has a ranch on the border and owns Marlin's, a saddle shop and western outfitters store on the city's main shopping street.

"I've been over there since all this happened, and I can tell you, it's different," Bohmfalk said of being in Agua Prieta.

"It's changed; it's quieter. There's not as much hustle and bustle, and there's a lot of people looking over their shoulder," he said. "It's something that is so palpable you can see it... It's almost like there's a storm coming or something. People are just not moving around."

His business plunged 50 percent in the days after the Cananea shootings.

Connie Whelan owns Paul's Jewelers in town and doesn't believe most Douglas residents are worried about a violence spillover.

"But people in Mexico who have lived there for years are afraid," she said.

The shootings have been what most customers who venture north from Mexico talk about, she said, many of whom live in Cananea. "We usually do a good graduation business, and we just haven't seen that business" this year because of the shootings.

For father-son ranchers Jack and John Ladd, the problem is the huge amounts of cash the cartels net from the drug trade. No honest police officer or public official can stand up to them, Jack Ladd said, because those who don't take payoffs end up being intimidated -- or killed.

Using a common Spanish reference for those facing the choice, John Ladd said, "'Plomo o plata,' silver or bullets."

This story appeared in The Daily Herald on page B2.