Out on patrol with heavily armed civilian vigilantes on Arizona's border with Mexico These volunteers say they are intercepting Mexican drug cartels.

 -- As President Donald Trump calls for a wall along the U.S.-Mexico border, a heavily armed group of citizen vigilantes are already carrying out a high-stakes game of cat and mouse there.

ABC News "Nightline" was there as Tim “Nailer” Foley was tracking footprints deep in the heart of some of the roughest and most remote desert Arizona has to offer.

“We believe there is a cartel scout watching our location just to our east,” Foley said. “We’re going to try to do a pinch on him. I got two headed south on this road and they’re going to cut into the wash, and we’re going to go to the top and push down.”

This paramilitary force says they are patrolling Arizona’s border to intercept illegal drug cartels from Mexico. They are heavily armed civilians, well organized and deeply committed. Once they spot someone they believe to be crossing the U.S.-Mexico border illegally, they chase them down and stay with them until law enforcement arrives.

Foley and his group say they have planted cameras along what they claim are routes used by drug mules and other undocumented immigrants making their way across the border into Arizona and beyond.

They see what they are doing as the living embodiment of what President Trump has warned about, the very reason he cites for his controversial executive order to build a wall.

But there is one problem with Foley’s group “border patrol” -- they aren’t U.S. Border Patrol officers. They are unauthorized armed forces that call themselves the Arizona Border Recon, AZBR, which “Nightline” followed after President Trump’s election.

Critics, including local law enforcement, say Foley’s group is dangerous. Tony Estrada is the longtime sheriff in Nogales, Arizona, one of the nation’s best-known and at times dangerous border crossings, and he said he would “rather not see” Foley’s group out here.

“The dynamics can be very dangerous if we have people coming in from the outside,” Estrada said. “They're not law enforcement don't have the authority to do immigration work.

“I think they're dangerous and in danger because nobody will vet them,” he continued. “Nobody's said, ‘Who are you? What’s your background? What’s your position on immigration? Are you a racist? What exactly is your purpose for being down here?’”

The U.S. Border Patrol said in a statement to ABC News in December that it “does not endorse or support private groups or organizations taking border security matters into their own hands as it could have disastrous personal and public safety consequences.”

The Southern Poverty Law Center goes so far to call them an “extremist group.” But Foley and his group of volunteers see themselves as patriots.

Foley, a 57-year-old former construction worker from Arizona, is the group’s founder. The group, which was formed in 2011, describes itself as an “intelligence-gathering organization” made up of “former military, law enforcement and private security Americans,” according to the AZBR website.

Foley says his group is more effective in capturing undocumented immigrants than the Border Patrol in these areas where the border doesn’t have much of a barrier -- some sections of the border are covered with X-shaped barricades known as “Normandy fencing.”

“There is no law here. That’s the point,” Foley said. “When they call us vigilantes, fine because it has been spun way out of line. If you look at the word vigilante, the first half is vigil. We’re doing a vigil here. The second half is ante, as in, ‘ante up, do your part.’”

His ragtag team of volunteers comes from all over the country, not just Arizona, to go out on patrols. Andy Poliakoff, one of his scouts who was armed, was there on vacation from his EMS job with a New York fire department. He said he believes Border Patrol and other similar agencies “are happy” that he and Foley’s volunteers “assist them.”

“I think that their hands are tied. They're not able to do what they really want to do,” Poliakoff said.

Foley says he background checks his volunteers, doesn’t allow drinking and routinely kicks out prospective members if they seem overeager and aggressive.

Jim Chilton, a third-generation rancher, owns and ranches 50,000 acres of land around the Arizona-Mexico border and said he was grateful to see Foley and his group out on patrol, whether they are authorized to do so or not.

“I like these guys,” Chilton said. “They're doing what citizens ought to do. They are helping the Border Patrol, helping all of us. We're here in a foreign occupied area with the cartel scouts on our mountains.”

The illegal border routes are nearly all natural trails carved though the desert by heavy rains. They are littered with signs of the smuggling trade, including empty black water bottles, which Foley said undocumented immigrants use to carry water so they don’t reflect light, and abandoned “carpet shoes,” a pad or piece of cloth that can be strapped to the bottom of their shoes so they don’t leave footprints behind.

But Foley and his group aren’t the only ones out patrolling these trails. Mario Ochoa is part of several humanitarian groups that leave food and water in the desert.

She said the elderly and children are routinely left for dead by human smugglers when the journey over the border becomes too hard for them to keep up. She showed a map covered in red dots indicating where bodies or human remains has been discovered in the area.

Ochoa worries Foley’s group are approaching groups of undocumented immigrants, who are often already being taken advantage of, with their guns and little accountability.

“There have been instances where people have gotten hurt,” Ochoa said. “They're not from our groups, not from them from these groups. But we have seen deaths out there that we're not sure who did it.”

A few years ago, Foley said he was accused of planting IEDs on these trails, allegedly targeting undocumented immigrants with makeshift explosives. He denies ever doing that, and said he wasn’t arrested or charged.

Foley shared a video of what he says shows him and his group helping what they call a “quitter,” an undocumented immigrant who has been separated from his group and wanted water and help, which they said they gave him before turning him over to the authorities.

“We were not a militia,” Foley said. “We are a nongovernmental organization. We don't advocate overthrowing the government or anything like that, and besides that, I live here so if you want to call me anything, call me neighborhood watch.”

And regardless of what President Trump says will happen with building a wall along the U.S.-Mexico border, Foley said a wall would slow down the number of undocumented immigrants coming into this country, “but it wouldn’t stop it.”

“You need to have somebody watching the wall,” he said.