FALFURRIAS, Texas – Just before sundown, a group of men cloaked in camouflage from the Texas Border Volunteers halts their all terrain vehicle, along a winding sandy road. As they make their way around the heavy brush, they circle around a pile of women's undergarments, which lay at the foot of a tree. In sections of land near the U.S.-Mexico border, this is known as a "rape tree." And for the residents of Brooks County, Texas, rape trees are popping up at an alarming frequency.

"I've had three rape cases in the last month," says Benny Martinez, the chief deputy at the Brooks County Sheriff's Department. "These guys are animals. There is an intimidation factor there. If they don't give into the brush guide, [the women] get beat up."

The group who found the "rape tree" are part of the Texas Border Volunteers, a troop of a few dozen private citizens who spend their own time – six or seven hours at a clip – weaving through the low-hanging honey mesquite trees and heavy Texas brush looking to stop immigrants from crossing into this land – and their country – illegally.

Symbols like the rape tree serve as a reminder to volunteers or anyone passing through of the escalating brutality "coyotes" are using to control immigrants they lead through this land. The tree is a trafficker's way of asserting power. But it also serves as a landmark for the border volunteers, allowing them to keep record of immigrant migration patterns through the brush.

When the Texas Border Volunteers head into the muggy summer night, each member gathers night vision goggles, flashlights, guns and folding chairs.

They run their shift as if they had been given marching orders from a commanding officer. They run their operation like a military mission, with code names like "gumball" and "pole cat" buzzing across their walkie-talkies.

For them this is a job, and they take it very seriously.

(Lauren Fox for USN&WR)

A team of Texas Border Volunteers gathers to finalize their plan before assuming their positions in the dense Texas brush and waiting for immigrants to cut across their fields.

Founded in 2006, the Texas Border Volunteers began as a citizen response to the growing amount of property damage occurring on the wide swath of private ranch land in Brooks County, Texas, an area 70 miles north of the U.S.-Mexico border. The county is home to the final border patrol checkpoint before human and drug smugglers funnel their inventory north. To get around the border patrol station, drivers often unload their cargo on desolate county roads and smuggle it through the dusty Texas fields for dozens of miles where they can hide among the thick landscape, sometimes for days.

The volunteers who patrol the ranch lands come from all walks of life; a mix of retired military personnel, educators, small business owners and computer technicians. While some live in the area, others come all the way from north Texas, or even out of state, to lend a hand. When they aren't on patrol, they sleep in a garage bunker or in tents on the ranch.

Veterinarian and rancher Michael Vickers, who has lived in Brooks County since 1986, founded Texas Border Volunteers out of desperation.

"They started cutting our fences and they broke into my house and tore up our water sources, breaking the floats off. And it has just been escalating," Vickers says. "The groups are getting bigger and the people are more combative."

Vickers' wife, Linda, never leaves the house without her cell phone, a gun and her four guard dogs.

Ranchers in the area say immigrants used to pass through without much of a trace.

"It used to be mostly Mexican peasants coming to look for work," Vickers says. "Maybe a couple times a week, they'd show up at my front door, ring the doorbell and ask if I had any work for them. If I didn't, I'd send them across the road or point them in the direction of work. They were very respectful. We'd give them water, we'd give them food."

Now, all along Route 281, bended fences, abandoned clothing and empty Gatorade bottles lay scattered on the road, evidence of groups passing through.

For some the high price is too much to pay to stay on the ranches. Elizabeth Burns, who owns a 38,000-acre ranch, moved off the land and into McAllen, Texas because the influx was growing too burdensome.

"We left because I didn't have any confidence. I saw the situation getting worse and I didn't see it getting addressed," Burns says. "I am not going to stay around and fight a transnational gang because you will lose. After five generations, it was not realistic for me to think this was going to be for the next generation.

In the fields, more canned food and trash lay beneath the trees. And more and more, skeletons are turning up in the high grass. Sometimes the dead bodies are still intact, still sitting uprights, swollen from the hot sun when ranchers and cowboys stumble upon them.

Vickers keeps dozens of blown-up photographs of their bodies in his office, which he uses to make presentations about border life.

The bodies are a disturbing yet routine part of life in Brooks County.

Vickers showing photos of people near death.

(Lauren Fox for USN&WR)

A sheriff deputy keeps a handful of photographs of dead people on his cell phone. As he scrolls through his phone, he stops at a photo of the body of Morales De Beltranena.

Like many immigrants, Beltranena died of exhaustion and was left behind by the man she paid to smuggle her across south Texas. Her brother had gone for help when Beltranena started struggling. By the time the deputy made it to her, she was dead.

"I just don't want these bodies to fall into collateral damage. They fall into deaf ears. All the bodies, the rapes, the property damage. It cannot be collateral damage. We can't have that. It's not acceptable," Martinez says. "This is a humanitarian issue. I know the federal government is looking more at a nationwide issue, rather than a specific issue that Brooks County is going through. After all, we are small in population, our voting is a small percentage and people are not going to get elected with Brooks County, but something has to be done."

Of the 463 bodies found in 2012 along the southwestern border, 129 of them surfaced in Brooks County. In Falfurrias, the largest town in the county, the bodies are buried at the local cemetery in simple wooden boxes under crooked aluminum signs. Waxy fake flowers adorn the graves of the unknown. It's rare for the county to make a positive identification because smugglers often confiscate the ID cards of their cargo. To help reunite the bodies with the families who may be waiting to hear from loved ones, Teams from Baylor University and the University of Indianapolis exhumed more than 50 bodies in May and sampled their DNA in hopes of finding more clues about where to return the bodies to.

"They are kind of at ground zero down here," says Kevin Cottrell, who made the journey from Austin to spend the weekend at the Vicker ranch.

The Texas Border Volunteers are simply back-up for U.S. Border Patrol. The goal of the volunteers is not to engage with the "criminal trespassers," as they call them, but alert border patrol of their findings.

In late June, as a border patrol drone zig-zagged across a ranch and patrollers on the ground searched for crossing immigrants, it was a volunteer who spotted a group of 10 immigrants running across a field. The volunteer called it into headquarters and helped border patrol apprehend five of the 10 immigrants.

"They catch a lot more because of what we do," Cottrell says.

Since 2008, when the volunteers began keeping logs, they have reported 2,142 illegal immigrants on ranch land in Brooks County. U.S. Border Patrol has been able to apprehend 1,180.

The number of immigrants passing through Brooks County is on the rise. The Rio Grande Valley saw a 65 percent increase in apprehensions in 2012. While border crossings appear to be decreasing nationwide, south Texas stands out.

Border patrol says it has dedicated historic levels of personnel, technology and resources to the border, but acknowledges they have seen an uptick in apprehensions in south Texas among immigrants from central America, including El Salvador, Guatemala and Honduras.

Border Patrol officials in the area say they work well with the border volunteers and appreciate the extra eyes and ears on the ground.

"The last month I was here, I spotted myself 33 [immigrants]," says Jean Swan, the only woman on patrol one weekend night. "Our nation is supposed to have a border and it is being breached constantly. The government is not doing anything about it and so some of us want to do it, anyway."