JEFFREY KAYE: It's a common scene along the U.S.-Mexico border: U.S. Border Patrol agents rounding up and arresting illegal migrants. No other stretch of America's border is as heavily fortified or as watched as the frontier that divides San Diego and the Mexican city of Tijuana, according to senior border patrol agent Kurstan Rosberg.
KURSTAN ROSBERG: I can tell you that in the San Diego sector for just San Diego and the sector-wide we arrested 126,000 people last year. So we are fairly busy here in San Diego.
JEFFREY KAYE: But not as busy as they used to be. In the early 1990s, images of migrants rushing immigration checkpoints here led to a crackdown. The federal government added border patrol agents, night scopes and motion detectors, stadium lighting, and, most visibly, higher and stronger fences.
In ten years, the number of migrants captured in the San Diego area has dropped by one-fifth, down from more than half a million in 1995, as illegal border crossers sought more remote routes.
Those who are caught here are processed, their fingerprints and photographs entered into a new database. Each day, U.S. Government buses deposit deportees at the border. Migrants pass through a gate back into Mexico, but many, such as Armando Martinez, vow to return.
ARMANDO MARTINEZ (Translated): It's always possible to cross over at one place or another.
JEFFREY KAYE: The still porous southern border has spawned new security initiatives ranging from the planned hiring of 1,000 additional border patrol agents to the use of an unmanned aircraft. But for anti-immigration activists and some members of Congress, those measures are not enough.
SPOKESMAN: Reports document the infiltration of this border by foreign nationals from terrorism-sponsoring countries including Iran.
JEFFREY KAYE: Using 9/11 as one justification, they say it's time for America to get even tougher in policing its southern border.
REP. DUNCAN HUNTER: 9/11 made the border a national security issue primarily, rather than simply an immigration issue.
JEFFREY KAYE: Republican Congressman Duncan Hunter of San Diego is a leading advocate of enhanced border security.
REP. DUNCAN HUNTER: Let's have control. Control should be first. Once you have control, you can shape policies.
JEFFREY KAYE: Hunter says respect for America's immigration policy has to start with a physical deterrent: A stronger and longer fence.
REP. DUNCAN HUNTER: I think that we have the biggest front door in the United States. That is, the biggest legal immigration door in the world is in the United States. And if people want to come into this country, they need to come in through the front door. And I think that's a reasonable request.
JEFFREY KAYE: To stop both migrant workers and potential terrorists from entering the U.S., Hunter favors building a fence along nearly the entire length of the almost 2,000-mile- long U.S.-Mexico border. Currently, only 70 miles of the international boundary is fenced.
Hunter scored a victory in December when the House of Representatives passed a border security bill. It requires the construction of nearly 700 miles of new border fences in California, Arizona, New Mexico and Texas at a cost of just over $2 billion.
As the legislation awaits Senate consideration, the Department of Homeland Security is about to begin a $35 million border security upgrade in the San Diego area. The money will be spent on new lights and on extending the triple-layered fence from nine to fourteen miles, all the way into the Pacific Ocean. One priority is to fill in a canyon along the border called "Smuggler's Gulch," a project long blocked by lawsuits over environmental concerns.
BORDER PATROL AGENT: The fence used to run up the hill. It has fallen down because of the erosion over the years. This is an easy place to walk in if we don't have the infrastructure and the fences here.
JEFFREY KAYE: Critics of plans to lengthen and strengthen border barricades dismiss arguments about keeping out terrorists as political opportunism, and they say too much border security goes against America's economic interest.
CLAUDIA SMITH: I don't think there is the will to hermetically seal the border.
JEFFREY KAYE: Claudia Smith is a migrant workers advocate with the California Rural Legal Assistance Foundation.
CLAUDIA SMITH: There is no bigger disconnect between what we say and what we do than in the subject of illegal migration. We all talk about wanting to keep the illegals out, but we want our illegal nanny, our illegal janitor. It goes against economic reality to erect a 2,000-mile fence.
JEFFREY KAYE: We met Smith outside of Tijuana's Casa Del Migrante, "House of the Migrant," a three-story Jesuit-run shelter. Here, migrants who either intend to enter the U.S. illegally or have been deported to Mexico can find a place to stay, as well as advice and food. When asked why they were making the trip, many here echoed the words of Luis Valdez. He's a father of three from central Mexico who was about to attempt his first illegal crossing into the United States.
LUIS VALDEZ (Translated): I have to go there to work. I need to get there to get ahead a bit in life for the sake of my family.
JEFFREY KAYE: Some deportees here told us that enhanced border security has discouraged them from trying to enter the U.S. again. Other migrants said they'd go around the fences and agents and try to cross the border in more remote and dangerous areas. The shelter distributes warm winter clothes to those planning a cross-border trek through the wilderness.
Rafael Rigos, who says he's almost died in the past crossing illegally into the U.S., is familiar with the perils.
RAFAEL RIGOS (Translated): The mountains make it easy to get lost if you don't know the way. But the biggest danger is lack of water. If you run out of water, you're done.
JEFFREY KAYE: Last year, nearly 500 people died trying to cross the deserts and mountains of the U.S.-Mexico border. It was the largest annual migrant death toll ever recorded by the U.S. Border Patrol.
CLAUDIA SMITH: So, construction of more and more fences, all it is going to achieve is shifting traffic from one place to another and pushing people into ever more remote and dangerous places -- not just the desert, but the most remote parts of the desert where the possibility of being rescued is virtually nil.
JEFFREY KAYE: But Congressman Hunter insists more physical barriers to illegal immigration at the border would end up saving migrants' lives.
REP. DUNCAN HUNTER: People would come to the fence, they wouldn't be able to get through, just like people can't get through the double fence here. So if they can't get through and they're deterred from crossing, they're not going to get out into that low desert in California and Arizona where the temperatures rise considerably above a hundred degrees in the summertime.
JEFFREY KAYE: At Casa Del Migrante, migrants say as long as desperation and poverty exist in Mexico and Central America, no amount of fences will stem the human tide of illegal immigration.
RAFAEL RIGOS (Translated): You can have a third or a fourth fence but it is not going to change anything. People are going to keep coming.
JEFFREY KAYE: In the U.S., even some advocates of tough border controls question the need for more fencing. The Border Patrol Agents Union wants stronger sanctions against employers who hire illegal immigrants. And the head of the Department of Homeland Security advocates more electronic surveillance: sensors, cameras, satellites and planes-- what he calls a "virtual fence."