By Mark Potter, Correspondent, NBC News

TUCSON, ARIZ. – On a helicopter inspection tour above the rugged mountains and vast desert in southern Arizona, Commander Jeffrey Self of U.S. Customs and Border Protection reflected on how much security has improved along the U.S.-Mexican border during his long career.

"After the vehicle barriers were built, and with the checkpoints going up, we're experiencing zero [undocumented immigrant] drive-throughs in an area where we were having 30, 40, 50 in a 24-hour period," he said, pointing to miles of vehicle barriers placed in the desert along the frontier.

U.S. Border Patrol has greatly reduced the number of cars and trucks loaded with people and drugs driving across the desert from Mexico into the United States. That, Self explained, has freed agents to focus their attention on immigrant and drug smugglers who walk across the border.  In the meantime, he added, authorities have also greatly reduced the number of hiking trails used by smugglers.

"In Arizona we have been very successful in increasing border security," Self said. "Over the course of many years now we've been resourced with tactical infrastructure, technology and personnel and they've been employed in a fashion that's gotten us greater results."

While conceding there are still many areas where drug and immigrant smugglers cross illegally into the U.S. -- often on private ranch land -- Self argued the threat has decreased dramatically and will continue to do so.

Mark Potter/NBC News

The U.S. border vehicle barrier used by authorities to stop trucks and cars from crossing the Mexican border in southern Arizona.

As the national debate over comprehensive immigration reform plays out, the question looms: just how secure is the U.S. border with Mexico? The answer appears to be mixed, with definite improvements nationwide and a downward trend in illegal immigration in most places – especially in the cities. But there are some areas, in rural Arizona and Texas, where residents insist the border is neither secure nor safe.

Gary Thrasher, a veterinarian and rancher in southern Arizona near Bisbee, says the rural border area where he works is actually less safe now than it was years ago, because of an increase in the number of armed drug and immigrant smugglers.

When the federal government increased security in the border cities, he said, it had the negative effect of forcing the smugglers to move to the large rural areas.

"The border statistically is securer than ever.  That means nothing,” he said.  “That's like saying we fixed this whole bucket, except for this hole down here.  You know it's still not going to hold water."

U.S. officials: look to the numbers 

Homeland Security Secretary Janet Napolitano frequently travels to the Southwest border and has made appearances before Congress where she has touted the recent improvements in border security and argued for passage of a comprehensive immigration bill.

"Fewer people are trying to emigrate illegally into this country than in four decades,” she testified before a U.S. Senate committee earlier this year. “What I know is that apprehensions are low, because attempts are low. Drug seizures, contraband seizures, all the numbers that need to be up are up."

In the year 2000, agents along the length of the Southwest border reported detaining 1,643,679 immigrants for allegedly entering the country without proper documentation.  Twelve years later, in 2012, that number had plummeted to 356,873, a decrease of 78 percent.

"San Diego and the Mexican border used to be the most lawless, violent places across the face of the earth with thousands of cross-border migrants on a given day,” said retired Gen. Barry McCaffrey, the former head of the Office of National Drug Control Policy. “We put in triple fencing and adequate Border Patrol and Coast Guard and it stopped."

Ranchers: rural border areas not secure

Critics of the administration's position on border security, however, say that while the overall apprehension numbers are down, they don't fully reflect the reality in areas where smugglers and immigrants still routinely make the illegal crossing into the United States from Mexico.

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An NBC hidden camera captures footage of border-crossers hiking across private U.S. ranch land in southern Arizona during late March.

On a small ranch near the border in southwestern Arizona, a mother of several children spoke under the condition of anonymity.  She fears what she described as an increase in drug and immigrant smugglers crossing her land by day and night.

"You're still having to pack a gun everywhere with you and make sure your kids can't go outside to play unless you are watching them." she said.  "The border is not secure. The Border Patrol doesn't have a very strong presence out here."

Texas police: a rise in immigrant smuggling

In the small town of San Juan, Texas, a few miles north of the Mexican border, Police Chief Juan Gonzalez toured some of the human stash houses his officers recently uncovered. They had been used to hide immigrants from all over the world who were smuggled across the border into the United States.

Gonzalez says his department has never dealt with as many undocumented immigrants as it encounters now. 

"In the past three years we've seen an increase.  And it's not a steady increase, it's a massive increase," he said.  "Too many people are getting through.  We've got too many holes in the border and we don't have enough manpower to make sure we secure the border."

About 75 miles north of the border, in Falfurrias, Texas, Benny Martinez, the chief deputy of the Brooks County Sheriff's Office, says his area is also deeply affected by a recent rise in illegal immigration. 

“The trending is going up,” he said.  “It hasn’t gone down at all, not here.”

Last year, officials and ranchers there found the bodies of 129 immigrants who died in the harsh terrain, presumably after crossing the border illegally.  Dozens are still unidentified and are buried in a local cemetery.  Some of the metal markers on the graves read, "Unknown Female" and "Unknown Remains."  One says, simply, "Bones."

Martinez does not believe the U.S.-Mexican border is at all secure in South Texas, given the rise in illegal immigration in Brooks County. 

"It's steady and I don't think it's going to go down, it's not going to happen anytime soon," he said.

PHOTOS: Border patrol faces surge in rural Texas border crossings

Ranchers like Linda Vickers, who lives just north of a Border Patrol highway checkpoint near Falfurrias, said she regularly sees, and often photographs, illegal immigrants cutting across her land as they try to evade the agents. 

“I’m seeing groups of 10, groups of 20 and I’m seeing them more often,” she said.

When asked about Obama administration claims that the border is more secure now, Vickers said that while it appears to be true elsewhere in the country, it’s not the case where she lives. 

“In the state of Texas, in the Rio Grande Valley, the border is not secure and I don’t think you’ll find a person, a real person, to say it’s secure,” she said.

Border patrol: South Texas a problem area

In South Texas, in the Rio Grande Valley sector, immigrant apprehensions rose 65 percent from the years 2011 to 2012 -- from 59,243 to 97,762, according to U.S. Border Patrol -- bucking the national trend of falling immigration numbers. 

This year, statistics reveal the Rio Grande Valley apprehension numbers have climbed even further, rising 55 percent compared to this time in 2012. 

Federal agents believe it reflects a recent increase in people fleeing the poverty, drug gangs and violence in Central America.

Privately, some agents say that, despite their great success in making more apprehensions, thousands of immigrants crossing the border illegally in South Texas still slip past them.

A majority of people involved in the security debate agree that most of the U.S. cities along the border are now much safer than they used to be and have much lower crime rates, thanks to high fences, increased monitoring technology and thousands of Border Patrol and other federal agents deployed there.  

But McCaffrey says U.S. officials need to do more for the rural areas.

“You have to give the Border Patrol and Customs and Border Protection the dollars and the technology to protect the American frontier,” he said.  “We’ve got to do it.  We owe it to the American people.”

This story was originally published on Thu May 2, 2013 11:29 AM EDT

By Al Henkel, Producer, NBC News 

FALFURRIAS, TEXAS --  Benny Martinez, the Chief Deputy of the Brooks County Sheriff’s office, gets out of his truck to give us a tour of the Falfurrias Burial Park.  

It’s depressing and sad. 

We’re just across the road from the Brooks County Fairgrounds.

Headstones are neat, well-maintained. Most date from the turn of the century, resting places for the founders of this region, hardscrabble ranchers who fought dust and heat and drought to build huge cattle ranches and oil fields in the fields of scrub brush and thorny mesquite trees.

Tucked in several corners, and now spilling over to the very edge of the property, are more than 100 lost souls. No headstones, just cheap metal markers, most with an ID number and some variation of the words: “Unknown Remains,” “Bones,” “Unknown Female.”

Last year the county buried 68 unknown people here, presumably undocumented immigrants trying to walk north, who didn’t make it.  Officials found a total of 129 people dead in the brush.

“Some of them are what you call OTM, 'Other Than Mexicans,' from Central America,” said Martinez.  “They die because either they get ill during the walk, or they weren't aware as to what it was going to take to do the walk.”

The “walk” is the trek from the border into the interior of the United States. Brooks County is well north of the Rio Grande. There is one main road, U.S. Highway 281, going through the area, which sports a Border Patrol checkpoint just south of Falfurrias.

Immigrants try to beat the checkpoint by walking around it. So far this year, 73 people have not made it.  The terrain is hostile at best, with few landmarks and little available water.  But it is the shortest route to the major cities, San Antonio, Houston, Austin, where immigrants can get lost in the population, and effectively disappear.

“They all want to get into the United States. This is the ‘land of the free’ and ‘get solid wages and make a life for yourself,’” said Martinez.  “The trending is going up, still up and maintaining.  It hasn't gone down at all, not here.  I’m guessing the flow is going to continue, or increase more.”

The Rio Grande Valley sector of the US-Mexico border is the most active spot in the nation for illegal immigrant traffic. Last year saw an increase of 65 percent in the number of people caught, and this year already shows a 55 percent increase over that.

“It’s overwhelming,” said Martinez, shaking his head. “And we are 75 miles north of the river.”

Martinez, a life-long resident of Brooks County, says talk of immigration reform, or hints of any policy remotely resembling amnesty, means more people trying to make “the walk,” with no way to stop it. 

“I don't see us really shutting this thing down.  I don’t see it.  Because I just think it's going to increase the volume,” he said. “It has to, it just makes sense it'll increase the volume of people coming across."

And with that increase in traffic, comes the possibility of more bodies found out in the brush.

In the past week, three more were found.  

By Mark Potter, NBC News correspondent

EDINBURG, Texas -- A cell phone call from a Spanish-speaking man who said he and others were locked in a house brought police to a dirt road on the outskirts of town.  There they found three small homes used as "stash houses" to hide 117 illegal immigrants, including 10 children, who had just been smuggled across the Mexican border into the United States.

One of the homes had barred windows and a padlocked door. "Approximately 50 undocumented people were inside the residence," said Edinburg Police Chief Rolando Castañeda. "There was no running water, very minimal food; it was pretty tough for them."

Of the 10 boys locked with the adults inside the house, officials said, nine were unaccompanied, meaning they were traveling without their parents or adult guardians. 

"They were being treated like animals," said Castañeda.  "There was a lot of desperation, there was a lot of fear in their eyes." 

All the children were taken to a local hospital, apparently suffering from dehydration.    

Child detentions double

Federal authorities said the number of children detained after illegally entering the United States is rising dramatically.

According to the U.S. Department of Health and Human Services, 8,327 unaccompanied minors were taken into the care of the Office of Refugee Resettlement from October 2011 to May 2012, after being picked up by the U.S. Border Patrol and Customs officials. That number is more than double the 4,016 unaccompanied migrant children detained during the same period last fiscal year. 

"Why this is happening we're still trying to figure out.  The kids are primarily coming from Central America, with Guatemala being the top sending country, El Salvador the second," said Wendy Young, the executive director of Kids In Need of Defense, a legal aid group.

Child advocates suggested many of these children, mostly teenage boys, are fleeing drug and gang violence in Central America. Another theory is that Central American and Mexican parents already living in the United States illegally, and who are afraid to return home and face the prospect of being caught by U.S. border officials, will often hire smugglers to bring their children to them, paying thousands of dollars per child. 

"Children are typically not the ones making the decision to come to the United States. Either someone is forcing them to leave their country or somebody is sending them," said Young.  "As soon as a child crosses the international border alone, that should be a red flag to us that the child is in need and to remember to treat these children as children first and immigrants second."

The U.S. Border Patrol reported that so far this fiscal year it has caught 15,590 unaccompanied immigrant minors, compared to 10,776 this time in 2011 and 13,267 this time in 2,010. 

Typically, children from Mexico apprehended after crossing the border are quickly turned over to Mexican authorities for a hasty return to their country.  Children from Central America are more likely to be taken into U.S. custody until their immigration status is determined and asylum claims can be adjudicated.

Shelters stretched thin

With the rapid rise in detentions of unaccompanied minors this year, federal authorities have had to scramble to find enough shelters to properly house them until relatives or guardians can be located to take custody.  One decision making headlines recently was the use of barracks at Lackland Air Force Base in San Antonio, Texas, to temporarily house some 200 immigrant children. 

Normally, U.S. Border Patrol agents would quickly hand over any unaccompanied children they catch to other authorities.  But recently they've had to spend a lot more time and resources caring for the children while officials try to find bed space elsewhere.

Stephanie Goodman, of the Texas Health and Human Services Commission, said state officials received pleas for help this year from the U.S. government. "We got a call in late February this year saying, ‘Hey, there's an influx of children coming in and we may need your help in expanding capacity,’" she said.  "In early March it was sort of a mad dash -- ‘Can you help us set up shelters in gymnasiums’ and things like that, because we have these kids sleeping in Border Patrol jail cells."

Child advocates worry that the rise in children held by the government will make it difficult to properly house all of them and to find enough volunteer attorneys to represent them before immigration judges. 

"It is really important to find a lawyer for them who can take the time, hear their story, develop trust with the child, develop a level of comfort and help them sort out do they want to stay or do they want to go home," said Young.  "The system is being stretched thin. We're in a crisis mode right now."

Both the U.S. Department of Health and Human Services and U.S. Customs and Border Protection declined requests from NBC News for on-camera interviews to discuss the issue.  When asked why they wouldn’t talk on the record, spokespersons for both departments said they were not allowed to explain the reasons.

Customs and Border Protection issued a written statement saying that while overall Border Patrol apprehensions have decreased 53 percent in the last three years, the Department of Homeland Security, "has experienced an increase in UAC (unaccompanied alien children) apprehensions compared to the same period in FY 2011.  This increase, however, is not inconsistent with historic migration trends and patterns, which are cyclical and vary month by month over a year."

A dangerous journey

Officials and advocates involved in the care of unaccompanied migrant children worry that the dramatically increased detention numbers mean that more children now are making the perilous trek from their home countries to the U.S. border. 

"It's often a very harrowing trip, a very dangerous trip for these children, and it can turn into a very abusive situation," said Young.  "We have a lot of girls who we work with who are victims of sexual violence, kids who've been robbed, kids who have been abused by (foreign) government authorities or others who just prey on them as they're on the move."

Mike Vickers, a South Texas rancher and veterinarian was surprised to find two girls, ages 10 and 13, walking with two teenaged boys through the brush and onto a busy highway. 

"They told me they were out of a group of 32 [illegal immigrants] and they got separated from that group," said Vickers. "There was dirt in their faces, you could tell they had been sleeping on the ground, sleeping in the dirt, sleeping in the brush and haggard. They were just absolutely spent."

Vickers called the local sheriff’s office and the U.S. Border Patrol and gave water to the children. "They probably didn't realize how close they were to being statistics and being dead," he said. 

It's a subject with which Vickers is quite familiar.  He has found numerous bodies of deceased illegal immigrants on his ranch, many of them victims of the extreme heat.  Last week, he encountered a 16-year-old migrant boy wandering on his property who took him to see the body of a young woman who had recently died there.  Her body was stretched out at the base of a tree. 

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Lorena Rodriguez, 13, spent six weeks traveling from El Salvador to the U.S. border with an older sister and smugglers. She now lives with family members in Boston, Ma.

A more fortunate child immigrant is 13-year-old Lorena Rodriguez, from El Salvador, who now lives with her adult sister and other family members in Boston after enduring a grueling six-week journey with another sister through Guatemala and Mexico.  After being helped by smugglers to cross the Rio Grande in an inner-tube raft near Hidalgo, Texas, Lorena and her sister were apprehended by U.S. Border Patrol agents and sent to shelters.

Attorney Carlos Maycotte, who has volunteered to represent Lorena and several other child immigrants, said the word "ordeal" doesn't adequately describe the discomforts and dangers involved in a trip with smugglers to the United States.  "It is very difficult, it is very long. They're not getting to sleep, they’re being smuggled basically like cargo, hidden in the back of trailers.  It's not an easy way to go."

With the statistics released by the federal government suggesting an increased number of unaccompanied children are making the perilous trip now, many officials and child advocates wonder if this is a temporary situation or if more long-term child-care solutions are required.

"You are still just putting the Band-Aid on the situation when you just expand the capacity for emergency shelter," said Goodman.  "You are not really dealing with the issue of why are these children coming in, and is there a better way to handle it?"