The bedraggled immigrants were picking their way through the boulders and scrub when a group of heavily armed men descended on them just short of the California- Mexico border. They corralled them in a cave and pointed their guns on the 10 men and one woman.

These lawless badlands in the hills east of Tijuana have long teemed with bandits and rapists, but these criminals demanded only phone numbers. They started calling the immigrants’ loved ones in Pomona, San Diego and Bakersfield: Send us money or we’ll shoot, they said.

The days-long kidnapping ordeal in May illustrates a growing trend as roaming gangs of well-organized, heavily armed gunmen turn their sights on illegal immigrants, making a treacherous journey ever more dangerous for people trekking north.

In the spree of kidnappings, which began about two years ago, gunmen hold people captive until family members in the U.S. send wire transfers of up to $5,000 to accounts in Mexico. Some immigrants are beaten; several have been killed, including a pair of brothers from Mexico City. Many straggle across the border and turn themselves in to U.S. Border Patrol agents. Others end up in migrant shelters in Tijuana, too frightened to report the cases to Mexican police.

Little was known about how the criminals operate until Mexican authorities dismantled two gangs in recent months, including the one in May, when 11 suspects were arrested after a shootout and a wild foot chase through the hills.

The arrests provided authorities with a rare glimpse into criminal networks whose reach stretches from the border to cities across the U.S. and Mexico. The gang was allegedly run by a career criminal from Nayarit and included a former Mexican army soldier. They admitted kidnapping more than 100 immigrants over 18 months, holding them in remote caves, makeshift camp and ranches.

“We threatened the families that if they didn’t pay we would kill the immigrants,” said Jose Flores Romero, the alleged ring leader, in his statement to detectives, referring to the abductions in May.

Authorities believe several gangs continue to operate. With a network of lookouts scattered at key points across 60 miles of rugged, isolated terrain, few immigrants slip by without them knowing about it.

“They know all the trails leading to the border, from Tijuana to Tecate and the La Rumorosa” mountain range, said Fermin Gomez, a Baja California assistant attorney general. “They know exactly where they’re going, how many are walking, and they’re all intercepted.”

The current situation resulted from a convergence of factors in the U.S. and Mexico that put increasing pressure on the traditional human smuggling groups in the area, according to authorities in both countries.

Organized-crime bosses in Tijuana, squeezed by a drug war, demanded higher payoffs, while U.S. authorities, adding fencing and staffing on the border, were making it more difficult to get immigrants through.

With a smuggling infrastructure already in place, it was easy and profitable for criminals to switch to kidnapping. Federal authorities in the U.S. immediately noticed the dangerous trend. Many immigrants began showing up at the border, seeking medical attention instead of eluding agents.

“They’re traumatized,” said Robert C. Rodgers, a supervisor for Immigration and Customs Enforcement in San Diego, which investigates smuggling groups in the area. “They jump the fence into the U.S. looking for help.”

The journeys start out straightforward enough. In Tijuana, recruiters scour fleabag motels that house immigrants waiting to cross the border. Offering safe passage, the recruiters transport the immigrants to staging areas in the ranchlands east of the city. Many of the recruiters and drivers are women who bring along their children to put the immigrants at ease.

At the staging areas, foot guides lead the immigrants into the mountains, down well-worn paths, into the hands of gunmen. “For the guides it’s a win-win proposition: They don’t have to cross the border, or risk being captured by the Border Patrol, and they still get paid,” one Mexican federal agent said on condition of anonymity because he was not authorized to speak with the media.

The deception doesn’t end there. One gang, arrested in April, would plant a member in the group of immigrants. When asked for a telephone number, he would immediately provide it and be rewarded with good treatment. Other immigrants, seeking to avoid beatings, would do the same.

“You’ve saved yourself,” the kidnapper told a gang member disguised as an immigrant, according to one victim in a videotaped interview with authorities that shielded his identity. “He acted like an immigrant, but we found out later that he wasn’t. We realized that they had never hit him.”

In the kidnapping incident in May, 11 immigrants were walking through the hills off the Tecate-Tijuana toll road when they were confronted at gunpoint. They were stripped, tied up and watched over by several gunmen while their families were contacted and ordered to wire $2,000 to gang associates in Mexico.

A couple of days later, Mexican authorities stumbled upon the gang during a routine patrol. In the ensuing gun battle, a Tecate deputy police chief was wounded in the leg by AK-47 gunfire. The group was eventually captured, and members led police to the alleged ring leader, Flores.

The highly publicized arrests have made immigrants aware of the dangers. Cupartin Sanchez, 27, interviewed at an immigrant shelter in Tijuana, said several smugglers had promised to get him across the border. He doesn’t trust any of them, so he’s going home to Guerrero.

“I’m not scared of the Border Patrol,” Sanchez said, “but I am scared of the kidnappers.”

richard.marosi@latimes.com