NOGALES, Ariz. » Flying low along the Mexican line in a Black Hawk helicopter, the United States Border Patrol officer saw surveillance towers rising above the cactus. He saw his agents’ white and green trucks moving among the mesquite, scouting for illegal crossers.

Far overhead, a remotely guided drone beamed images of the terrain to an intelligence center in Tucson. Pilots cruised in reconnaissance planes carrying radars and infrared cameras that could distinguish a migrant with a backpack from a wild animal from many miles away.

Sabri Dikman, the patrol’s executive officer for this region, liked what he saw — and what he did not see. If any illegal migrants were trying to make their way into the United States, he said, the chances were very good that agents would find and detain them.

"We know what we’re dealing with and where we are dealing with it," Dikman said. "In some manner we have the capacity to observe every part of the border of Arizona."

As Congress debates a broad overhaul of the immigration laws, including a pathway to citizenship for 11 million illegal immigrants in the country, skeptical lawmakers are asking if the Southwest border is secure enough to withstand any new wave of illegal crossings that might be spurred by a legalization program, or by new growth in the American economy.

Officers who guard the line say the border is more secure in most places than they have ever known it. They say they are in a strong position to hold off an illegal surge, and to show why they point to Arizona, once the busiest and most contentious border battlefront. To the east, in Texas, agents are still struggling to stop persistent migrants in hundreds of miles of varying and penetrable terrain. But in Arizona, every available measure shows steep declines in the number of people making it across, figures that border agents say demonstrate what they can accomplish.

In Congress, many Republicans recall that an amnesty in 1986, which was supposed to solve illegal migration, was followed by an even larger unauthorized influx. A bipartisan group in the Senate is working on a proposal that would require measurable gains in border security before immigrants would be allowed to proceed onto a path to full citizenship.

But to border officials here, Congress seems to be behind the times, failing to notice that they have already made many of the enforcement advances that lawmakers are seeking. Since 2005, the number of patrol agents in the Southwest has nearly doubled, to more than 18,000. Customs and Border Protection, the parent agency of the Border Patrol, has built its air wing to more than 260 aircraft. It acquired an array of technology, including ground sensors and aerial detection devices developed by the Defense Department in Iraq and Afghanistan, and created a military-style command structure with expanded intelligence operations to coordinate agents on the ground.

While tightening the border, officials also created a system of penalties for those who get past it, making it far more likely that migrants who were caught would be jailed, prosecuted as criminals, or expelled from the country far from the place where they tried to enter.

Border officers, once a beleaguered force overwhelmed by illegal flows, have acquired a bit of swagger.

"It’s all of these things coming together that have really given us the capability to bring the border to a level of security that is, in my opinion, unprecedented," said David V. Aguilar, the agency’s commissioner, who will retire this month after 34 years on the borders. "We haven’t seen this lack of activity in 25, 30 years," he said.

In Arizona in 2005, more than 577,000 apprehensions were made of migrants crossing illegally, a surge that sparked a political furor in the state. Border officials declared that they would make a "last stand" here.

Last year in Arizona, the apprehensions (an imperfect but still useful indicator of unauthorized flows) dropped to 126,500, a 78 percent decline from 2005, a sharper decrease than seen elsewhere along the border. The recession in this country and drug-trafficking violence in Mexico also contributed to the falloff, here and along the length of the border. But in 2005, about half of all apprehensions in the Southwest were made in Arizona. Today, it is closer to one-third, with the largest share now shifted to Texas.

Dikman remembers the bad old days, when the agents’ rule of thumb was to open a formal deportation only after an illegal crosser had been caught and expelled to Mexico 18 times.

Now, Customs and Border Protection is adjusting its strategies to the reality of greatly reduced numbers, but more troublesome adversaries.

Jeffrey Self, the commander of the agency’s Joint Field Command for Arizona, based in Tucson, said migrants rarely attempt to cross these days without hiring a coyote, or smuggler, and those guides are generally linked to narcotics organizations in Mexico.

"It’s pretty much all organized smuggling at this point and it’s pretty much all controlled by the transnational criminal organizations that operate over there," Self said.

Border officials worry there may not be much more they can do with enforcement alone to further reduce illegal crossings and sharpen the focus of their operations on criminal threats, without other changes in the immigration equation.

"If this country has a continued demand for labor by way of our market economy," said Aguilar, the commissioner, "you have a baseline draw for illegal immigrants."

He said if Washington created more legal channels for migrants who want to come to this country primarily to work, it could significantly cut the numbers of people attempting to pass illegally, freeing border officers to pursue traffickers and other criminals.

"Now that would be a tremendous force multiplier," Aguilar said. Lawmakers are considering proposals for some form of temporary guest worker program, and also measures to give legal status particularly to migrant farmworkers.

Officials said border gains could also be affected if Congress imposed deep budget cuts. Last week Aguilar announced the beginning of 14-day furloughs for his agency’s employees under the sequester.

TWIN CITIES

Here in Nogales, a laid-back border town that surrounds the largest port of entry in Arizona, Border Patrol vehicles drive freely along the dirt road along an 18-foot barrier of metal poles that divides the U.S. city from its Mexican twin, also called Nogales. Until recently, agents had to keep a distance. But Leslie Lawson, the top Border Patrol agent in Nogales, Ariz., said the barrier was an example of a small tweak that made a big difference.

A lower fence built in the 1990s had been covered with mats to prevent Mexicans from seeing through. Smugglers would brazenly send people or narcotics over the fence, then hurl rocks down on agents from hills on the Mexican side while the migrants slipped away into nearby gulches. In 2011, Lawson’s sector had more rock assaults on agents than any other in the country — 170 in all.

Last year three miles of the old fence was replaced with the poles, set just far enough apart to see through. Agents can spot migrants before they approach the fence. Rock assaults are down 60 percent.

"Every day the smugglers will change their tactics," Lawson said. "They are always watching us. So we are always changing our tactics."

With tighter security along the line, more undocumented foreigners are trying to come through the border station. But recent changes have made it more difficult for smugglers to pass their clients with forged papers. Instead, officials said, smugglers rent out valid U.S. documents, and encourage their clients to make themselves look as much as possible like the people in the photographs.

One day in late February, an elderly Mexican woman and a Mexican farmworker were stopped at the Nogales crossing after they presented documents that were legitimate but did not belong to them.

A sharp-eyed customs inspector at the pedestrian walkway also singled out an 18-year-old Mexican girl who had a valid border crossing card but also a nervous twitch. A pat-down revealed that the teenager, Selena Andrea Perez Cruz, had stashed nearly a pound of heroin in her bra.

Perez Cruz was turned over to federal authorities in Arizona for narcotics prosecution. But officers were not done with the other Mexicans either. Part of the Customs and Border Protection strategy is to ensure that almost all migrants who are detained face a penalty on this side before being deported — what officers call their consequence delivery system. Commander Self said some penalty was applied to at least 90 percent of illegal migrants apprehended at the state’s border.

In a holding cell, the Mexican grandmother, who said she had hoped to reunite with relatives in Arizona, was formally booked. Her record would remain in federal databases after she was expelled. If she was ever caught trying to enter the United States illegally again, she could face felony charges and months, if not years, in prison.

Other forlorn Mexicans stopped in Arizona were waiting to be deported through a transfer program, another one of the consequences. They would be taken by bus three days east and expelled at Del Rio, Texas

Officers use the transfers "to break that smuggling cycle, to separate the smuggled aliens from the smugglers and the guides," Self explained.

Some migrants detained in Nogales would be turned over, under a consequence program called Operation Streamline, for criminal prosecution. In Yuma, which has applied the program for six years to virtually every illegal crosser, the fast-track cases have clogged the federal courts and drawn a host of complaints from legal rights groups.

But their impact is unmistakable. In 2005, more than 138,000 apprehensions were recorded in Yuma. By last year, the number had decreased to 6,500, a decline of 95 percent. Since authorities began to apply the consequence system five years ago across the southwest border, the numbers of deportees who try to cross again within a year has consistently declined.

One defender in Nogales of the border agency’s progress is the mayor, Arturo Garino. He says his city has become one of the safest in Arizona, with one murder in the past six years.

"We used to have street chases all the time," he said in an interview in City Hall. "Now all those things are gone, something you don’t even hear about." He said lawmakers in distant Washington should concentrate on fixing the immigration system.

"I don’t care how many more Border Patrol you have," Garino said. "You can’t secure this border any more than it is right now and that is a fact. Do we want to line them up, hand in hand, all the way to Texas?"

TAKING A TOLL

Beyond urban areas, however, control is spottier. Ranchers close to the border report that they often see migrants on their property, and they do not feel safe.

But added staff and technology have greatly expanded what Customs and Border Protection officers call their situational awareness. On a wall of video screens in his command center in Tucson, Self can summon aerial images of vast swaths of borderland. Thousands of agents are patrolling in vehicles, on all-terrain vehicles and on horseback. When incidents arise, the closest ones can be quickly dispatched.

But greater efficiency has exacted a human toll. Migrants have been pushed ever farther toward the rock faces and remote valleys of the border’s most deadly landscape. Migrants pay smugglers higher fees, but with security tight, the coyotes no longer feel any obligation to finish the job of guiding them to the United States.

According to data compiled by The Arizona Daily Star, last year 172 human bodies were found in the state’s border desert.

In late February, on a mountain flank west of the border outpost of Sasabe, 19 migrants tried to slip past the Border Patrol’s deployment. A rare blizzard had brought a biting freeze, frosting the saguaro cactuses with snow. With only light jackets and plastic ponchos, the men, from Mexico, Honduras and Guatemala, spent three nights in the cold while their hired coyotes searched for an opening between agents on horseback and surveillance planes overhead.

On the fourth morning, inside U.S. territory, the men arose to discover that their guides had fled.

Utterly lost, they chose to turn back. After another six hours wandering through cactus brush into Mexico, they happened upon a Mexican government rescue group. The men were mute with hunger and thirst.

After a gulp of water, Helio Leon Reyes, 31, a Mexican from the state of Guerrero, began to cry. The cold, he said in Spanish, "was too much to bear." His feet were still stiff with cramps and his money was lost to the smugglers.

Leon said he had hoped to make it to New York, where a brother lives. But he decided on that day that his trip to the United States was over. He would go back to his wife and children. "This is too complicated," he said.

© 2013 The New York Times Company