AGUA PRIETA, Mexico — "My wife, my son — I have to get back to them," Daniel kept telling himself, from the moment he was arrested in Seattle for driving with an expired license, all the way through the deportation proceeding that delivered him to Mexico in June.

Nothing would deter him from crossing the border again. He had left his hometown at 24, he said. Twelve years later, he spoke nearly fluent English and had an American son, a wife and three brothers in the United States. "I’ll keep trying," he said, "until I’ll get there."

This is increasingly the profile of illegal immigration today. Migrant shelters along the Mexican border are filled not with newcomers looking for a better life, but with seasoned crossers: older men and women, often deportees, braving ever-greater risks to get back to their families in the United States — the country they consider home.

They present an enormous challenge to American policy makers, because they continue to head north despite obstacles more severe than at any time in recent history. It is not just that the American economy has little to offer; the border itself is far more threatening. On one side, fences have grown and American agents have multiplied; on the other, criminals haunt the journey at every turn.

And yet, while these factors — and better opportunities at home — have cut illegal immigration from Mexico to its lowest level in decades, they are not enough to scare off a sizable, determined cadre.

"We have it boiled down to the hardest lot," said Christopher Sabatini, senior director for policy at the Council of the Americas.

Indeed, 56 percent of apprehensions at the Mexican border in 2010 involved people who had been caught previously, up from 44 percent in 2005. A growing percentage of deportees in recent years have also been deported before, according to Department of Homeland Security figures.

For the Obama administration, these repeat offenders have become a high priority. Prosecutions for illegal re-entry have jumped by more than two-thirds since 2008. Officials say it is now the most prosecuted federal felony.

President Obama has already deported around 1.1 million immigrants — more than any president since Dwight D. Eisenhower — and officials say the numbers will not decline. But at a time when the dynamics of immigration are changing, experts and advocates on all sides are increasingly asking if the approach, which has defined immigration policy since 9/11, still makes sense.

Deportation is expensive, costing the government at least $12,500 per person, and it often does not work: between October 2008 and July 22 of this year, Immigration and Customs Enforcement spent $2.25 billion sending back 180,229 people who had been deported before and come back anyway. Many more have returned and stayed hidden.

Some groups favoring reduced immigration say that making life harder for illegal immigrants in this country would be far more efficient. They argue that along with eliminating work opportunities by requiring employers to verify the reported immigration status of new hires, Congress should also prohibit illegal immigrants from opening bank accounts, or even obtaining library cards.

"You’d reduce the number of people who keep coming back again and again," said Bob Dane, a spokesman for the Federation for American Immigration Reform. The alternative, says Doris Meissner, the country’s top immigration official in the mid-1990s, is to accept that illegal immigrants like Daniel "are people with fundamental ties to the United States, not where they came from."

"Our societies are so deeply connected," Ms. Meissner said, referring primarily to the United States and Mexico, the main source of illegal immigrants. "And that is not reflected at all in policy."

The administration acknowledges that immigrants like Daniel are rooted in the United States and typically have otherwise clean criminal records. But under its new plan introduced in August — suspending deportations for pending low-priority cases, including immigrants brought to the United States as children — repeat crossers are singled out for removal alongside "serious felons," "known gang members" and "individuals who pose a clear risk to national security."

Administration officials say they are trying to break the "yo-yo effect" of people bouncing back, as mandated by congress when it toughened laws related to illegal re-entry in the 1990s.

But some experts argue that this commingling actually undermines security. After a decade of record deportations, critics argue, it has become even harder to separate the two groups that now define the border: professional criminals and experienced migrants motivated by family ties in the United States.

"If you think drug dealers and terrorists are much more dangerous than maids and gardeners, then we should get as many visas as possible to those people, so we can focus on the real threat," said David Shirk, director of the Transborder Institute at the University of San Diego. "Widening the gates would strengthen the walls."

Crime and the Border

The border crossers pouring into Arizona a decade or two ago were more numerous, but less likely to be threatening. David Jimarez, a Border Patrol agent with years of experience south of Tucson, recalled that even when migrants outnumbered American authorities by 25 to 1, they did not resist. "They would just sit down and wait for us," he said.

Over the past few years, the mix has changed, with more drug smugglers and other criminals among the dwindling, but still substantial, ranks of migrants.

The impacts are far-reaching. In northern Mexico, less immigration means less business. Border towns like Agua Prieta, long known as a departure point, have gone from bustling to windblown. Taxis that ferried migrants to the mountains now gather dust. Restaurants and hotels, like the sunflower-themed Girasol downtown, are practically empty. On one recent afternoon, only 3 of the 50 rooms were occupied.

"In 2000, we were full every day," said Alejandro Rocha, the hotel’s manager.

New research from the University of California, San Diego, shows that crime is now the top concern for Mexicans thinking of heading north. As fear keeps many migrants home, many experienced border guides, or coyotes, have given up illegal migration for other jobs.

In Tijuana, across the border from San Diego, one well-known coyote is now selling tires. In Nogales, the largest Mexican city bordering Arizona, power has shifted to tattooed young men with expensive binoculars along the border fence, while here in Agua Prieta — where Mexican officials say traffic is one-thirthieth of what it once was — the only way to get across is to deal with gangs that sometimes push migrants to carry drugs.

It is even worse in Matamoros, across from Brownsville, Tex. Just standing at the border fence brings out drug cartel enforcers demanding $300 for the right to pass. Migrants and the organizations that assist them say cartel lieutenants roam the shelters, looking for deportees willing to work as lookouts, earning $400 a week until they have enough to pay for passage north.

"I was thinking about doing it, too," said Daniel, looking down. "But then I thought about my family."

American law enforcement officials say the matrix of drugs, migration and violence has become more visible at the border and along the trails and roads heading north, where more of the immigrants being caught carry drugs or guns — making them more likely to flee, resist arrest or commit other crimes.