NOGALES, Ariz. » A U.S. immigration agent bounded up the steps of a bus about to cross the U.S.-Mexico border recently and demanded to see the papers of all those aboard.

"Papers!" he shouted, eyeing passengers warily as he walked up and down the aisle.

Such checks are not surprising given all the attention focused on illegal immigration these days. But this bus full of migrants was leaving the United States, not entering it.

A raft of immigration laws in Arizona and other states is designed to make life so difficult for illegal immigrants that they pack their bags and head home. But the reality on the border is that departing the country has become more complicated than ever — leading some people to worry that the outbound checks could not only dissuade illegal immigrants from leaving the country but also place them in a kind of no-win limbo, reviled if they stay and potentially arrested if they try to leave.

It used to be that entering Mexico, whether it was from San Diego, El Paso, Texas, or here in Nogales, was a cakewalk, with no scrutiny on the U.S. side of the border and next to none on the Mexico side. But efforts by the Obama administration to reduce the flow of guns and drug money heading from the United States to Mexico have changed that in recent years.

Agents regularly hop aboard southbound buses, a common way for migrants to return to their towns and villages. At permanent checkpoints set up at border crossings, they also stop southbound vehicles and confront pedestrians going south on foot.

In questioning people leaving the country about illegal contraband, agents frequently find migrants who are not engaged in smuggling but do not have permission to be in the United States. Some with clean records are let go. Others are fingerprinted and photographed for illegal entry and only then allowed to go on their way. Once they are in the government’s database, they face more stringent penalties if they are caught in the United States again.

Immigrants who are found to have criminal records face more aggressive treatment. They are likely to be arrested and then formally deported.

The intent, officials say, is not to discourage illegal immigrants from leaving. Rather, it is to stem the flow of contraband. In a recent weekly report from Arizona, Customs and Border Protection said it had seized $22,102 in cash being smuggled out of the state from July 18 to July 24. During the same period, six weapons and 5,943 rounds of ammunition were recovered. Agents detained 1,606 illegal immigrants, although that included those who were both coming and going.

In interviews, departing immigrants offered a variety of reasons for leaving Arizona. Tough laws and law enforcement sweeps made life less livable. The economic downturn made it tougher to make ends meet. Then there was also a host of personal concerns. For Analleli Rios Ramirez, 24, it was the death of her brother-in-law in Cuernavaca that prompted her and her husband to decide to live closer to relatives.

"We just decided we wanted to live in our own country," said Rios, who was the assistant manager of a pretzel shop in a mall near Phoenix.

Some question the sense of checking the papers of migrants who are leaving anyway. The criticism comes from those who consider illegal immigrants to be outlaws and those who sympathize with their struggle to improve their lives.

"Why do we want to spend resources apprehending people who are removing themselves anyway?" asked Jennifer Allen of the Border Action Network, a human rights group based in Tucson that aids immigrants in southern Arizona. "I’ve heard of people wanting to leave the country and wondering if they should risk it. It’s in the forefront of people’s minds when they’re deciding to leave."

The possibility that a government policy might be discouraging illegal immigrants from leaving has led even some groups who favor tighter immigration controls to think twice about the southbound scrutiny.

"This is about the only situation we would ever advocate that our immigration laws be waived," William Gheen, president of Americans for Legal Immigration PAC, said last year in a statement calling for the Obama administration to ease its southbound immigration checks. "We want to encourage the illegals to leave America on their own, and thus we ask Obama to provide them safe passage out of America."

Making it difficult to leave the country, Gheen said, might prompt some migrants instead to leave Arizona or other states with tough immigration laws for more hospitable parts of the United States.

Despite the second guessing, the administration said the policy made sense.

"We’re not trying to discourage anyone from leaving, but we do want to send the message that there are consequences for breaking immigration laws," said an administration official, who was not authorized to speak publicly on the matter.

Although thousands of migrants have been detained heading south, officials said they could not break down how many were stopped because they were in the country illegally versus those stopped for smuggling violations.

Some immigrants said they were confused by the policy.

As she prepared to cross the border recently and join her husband who had crossed months before, Rios grew anxious, knowing that she did not have her papers in order and that she might be detained. She had entered the country illegally more than a decade ago, as an 11-year-old child clutching her mother’s hand. Now she was returning to a country she barely knew.

"I thought this is what Arizona wanted, for me to leave," she said as she packed her things in Chandler, Ariz., before heading south. "And I have to worry about them catching me on the way out."

It turns out that she and her overstuffed pickup truck crossed from Nogales, Ariz., into Nogales, Mexico, without a hitch.

Immigration officials say they cannot check everyone and use discretion as they survey the documents of departing migrants. Rios’ mother, Teodora Martinez, had left months earlier and did not have papers when an agent hopped aboard her bus. She presented an identification card issued by the Mexican Consulate in Phoenix, which did not prove legal residency. Her husband, Cesar Valle Martinez, had shown a fake ID.

The agent raised his eyebrows as he surveyed their papers and then huddled with a colleague who had also entered the bus. The couple was traveling with several young children, though, and they had U.S. passports.

"Go on," the agent said finally, handing back the documents, exiting the bus and letting the family return to Mexico.

© 2011 The New York Times Company