MCALLEN, Texas—When they last spoke, Luis Fuentes heard relief in his wife Marilu Noely Alas Santos's voice. She had her feet firmly planted on U.S. soil. And after a 2,000-mile journey from El Salvador, she and her sister Reina Carolina had just crossed the Rio Grande into Hidalgo County, Texas. After being extorted along the way, the women thought the most treacherous leg of their journey was behind them.

It was the last time Luis spoke with his wife.

Within hours, as Marilu and Reina loaded into a car that had come to pick them up and take them to Houston, Texas, Reina was drug by her hair into a separate vehicle and driven to the Palace Inn in Alamo, Texas where she says was raped in Room 116 for two-and-a-half days until she escaped.

Her sister, Luis' wife, is still missing. Her whereabouts are unknown.

"I am very hopeful that I am going to find her alive. I am going to get an answer for my children. There is no stopping me until I know," he says.

He spends his days now in New York as a single parent on hold with law enforcement, sifting through records and scouring the Internet for any signs of his missing wife. Every time the phone rings, he rushes to answer it in hopes it's Marilu on the other end.

Luis and Marilu met in 2006 in the United States where they lived together until Marilu's father fell ill and she returned to El Salvador to care for him. As an immigrant who had entered illegally and had already been apprehended by border patrol once before, Marilu sought to return to the U.S. in 2012 with the help of a human smuggler.

"I told her a million times it was dangerous," Luis says. "It used to be a lot easier. Now it is just crazy."

The landscape immigrants must endure to attain the American dream has radically transformed in recent years. As the number of border patrol agents along the southwest border has more than doubled since 2001, crossing from Mexico into the U.S. has become a nearly impossible feat to accomplish alone.

"Human smuggling is a function of border patrol," says Rey Koslowski, an expert on human smuggling and a professor at the University at Albany. "Zero border control and there is no need to pay anyone to help you cross. If you increase the difficulty of crossing the border, that leads to a market for people to help you evade whatever those obstacles may be."

As demand for smugglers has spiked so have the costs, and what used to be a relatively inexpensive journey shepherded by mom-and-pop smugglers has transformed into a lucrative business. In the mid 1990s, immigrants might pay $50 or $100 for a "coyote" to float their things and guide them across the Rio Grande. Today, the United Nations estimates, immigrants traveling from Central America pay anywhere from $6,000 to $10,000 to sneak through Mexico into the United States. Immigrants crossing just from Mexico still pay an estimated $2,000.

With migrant smuggling becoming more profitable, transnational gangs and drug cartels have jumped at the chance to get a piece of the pie. Crime groups like the brutal Los Zetas have elbowed their way into the business, terrorizing migrants who attempt to cross the border without paying off their network and abusing those in their care.

In 2010, members of the drug gang executed 72 immigrants in Mexico when they refused to join the organized crime unit and declined to pay a ransom fee. When police raided the Mexican ranch where the migrants were shot, they found an arsenal of assault rifles and military combat gear including bullet proof vests.

The violent turf wars being waged in Mexico between cartels are no longer just for drug territory, but for human smuggling routes.

"That has changed the morality of smuggling. Smuggling has become human trafficking. Migrants are coerced, threatened and killed along the way," Koslowski says.

The U.N. estimates transnational crime organizations make nearly $7 billion annually smuggling migrants from Mexico. With so much money to be made, the groups are emerging as sophisticated forces against other crime units and against U.S. law enforcement.

Some local agents in border towns complain the smugglers' techniques, gear and weapons are quickly outpacing theirs. Agents say hiding within the colonias, the Mexican-American neighborhoods along the Rio Grande where dilapidated trailers and poor farm workers reside, gang and cartel members are monitoring the whereabouts of law enforcement vehicles and agents.

"When I am doing ride-alongs with agents, you see people calling out our movements," says Rosendo Hinojosa, the chief border patrol agent for the Rio Grande sector. "Anytime they move narcotics or migrants, they have scouts. They are trying to defeat our defense."

While an immigrant journey may be arguably less predictable in Mexico with gang and cartel violence along the way, the terror doesn't end when immigrants arrive on the U.S. side of the border. Lupe Trevino, the Hidalgo County Sheriff, says he's overwhelmed by the migrant abuses that spill across the border.

Throughout the county, smugglers have established a network of stash houses, places where human smugglers keep immigrants once they've crossed the U.S.-Mexican border.

On a recent – and typical – Monday, the Hidalgo County Sheriff's tactical unit get out of their vehicles and surround a small, single-level apartment. The windows, covered in tinfoil, are locked. The door, dead-bolted. The officers feverishly began knocking and talking to the more than 30 individuals they suspect are inside, but their pleas are met with silence.

After 15 minutes, the officers abandon the exercise and drive away.

Stash houses are becoming a real problem in Hidalgo County. The resting places, sometimes torture chambers or exploitation centers, are hidden throughout the urban centers and rural outposts.

"It's almost like a warehouse. They treat these individuals like commodities," Trevino says. "The problem is that a lot of those [undocumented] folks are being victimized while they are there. The women are being assaulted and the men are robbed. We are averaging about three human stash houses a week now."

Sometimes, Trevino says the police will uncover 100 people in a three bedroom, single bathroom home. The smugglers feed occupants cold, canned food and crowd them onto old mattresses to sleep.

"The living conditions are deplorable. These places are not fit for human occupancy," Trevino says.

But the stash houses are just one horrifying leg of a more perilous journey.

When they leave the stash houses, migrants are stuffed like sardines in the back of semis, in the dashboards of pick up trucks, in the floorboards of Suburbans or in the trunks of small cars and driven up north. But border checkpoints as far as 60 miles north present another dangerous juncture.

In order to avoid the checkpoint, many smugglers will drop their migrants off on unmarked county roads, tell them the walk is just a mile or two, and send them on their way with a guide.

The walk is much longer than that, however. Many immigrants wind up walking 15 miles or more through the thick brush and along sand paths that feel heavy beneath their already weary legs.

If they cannot keep up, they are left behind to die of dehydration in the muggy fields. Sometimes, if they refuse to carry the 70-pound packs of marijuana for their smugglers, they are beaten. Women are raped and their undergarments are left hanging in the mesquite trees.

In Brooks County, Texas, the site of one of the checkpoints, 129 bodies were found last year. Already, 33 have been discovered this year.

"This area is the hot spot now," says Linda Vickers, a rancher who has had more than one run-in with the groups and bodies in the area.

The county buries the bodies in the local cemetery and places thin metal signs to mark the graves of the unknown.

"They are humans. That is what we all are," says Benny Martinez, the chief deputy at the Brooks County Sheriff's Department. "Who would blame them for wanting to get out of their country with crime as high as it is. They just want to feel comfortable. They just want to be looked at as human beings."

Back in New York, Luis says he has found it difficult to get law enforcement agents to see his wife as a person worth finding. He says some local police agencies have been slow to investigate and are skeptical of his story. In a world where city, county and federal law enforcement agencies' duties are blurry, he says it's easy to get the run around.

One local law enforcement agent who has worked closely with him, but was not authorized to speak on the record, confirmed that the case fell into ICE's jurisdiction. He also confirmed that his agency had taken down information about Marilu's disappearance and had collected photographs, and passed them along to law enforcement agencies throughout the state.

"It is pretty sad. I am pretty much one of the only [local] agents that takes some kind of report and I send them to Brooks County," he said. Brooks County is the area where many immigrants die of dehydration during their journey.

"When I get calls like this, they are so genuine. I know what is happening. Everyone knows what is happening. We know there is a criminal element that is doing this to the women," the officer said. "You hear countless stories. My wife. My sister. My aunt. My mom. It is sad. God knows how many bodies are out there that have not been recovered yet."

With such a high volume of police reports of lost immigrants, Luis says he has hired a private investigator to help him find his wife.

"Every day I ask God to help me," he says. "Seeing my children is the thing that gets me up every day. I cannot fall. I am a mother and a father to my children now."