Not every illegal immigrant in the United States snuck across the border. A very large number, perhaps as many as 5.5 million, entered legally with visas and then never left.

But unlike the hundreds of thousands of illegal immigrants apprehended at the border every year, very few visa violators are ever caught.

The Border Patrol's Tucson sector, the busiest in the nation, logged 112,488 apprehensions last fiscal year. In comparison, federal agents in Arizona tracked down and arrested 27 people who had overstayed their visas.

Visa violators represent nearly half of the 11 million illegal immigrants in the country. But they have been largely ignored amid a national clamor to secure the border, fueled in part by Arizona's tough new immigration law, the killing of a southern Arizona rancher and worries that cartel violence in Mexico could spill into this country, analysts and experts say.

"It's not that we have too much emphasis on the border. We still need enforcement on the border. The problem is not enough attention to the other issue," said Michael W. Cutler, a former senior agent with the Immigration and Naturalization Service, which became Immigration and Customs Enforcement.

In the past five years, the number of Border Patrol agents stationed along the U.S.-Mexico line has doubled, to more than 20,000 people. That's the highest level of staffing in the Border Patrol's 85-year history. And Arizona politicians including Republican Gov. Jan Brewer, Republican Sens. John McCain and Jon Kyl, and Democratic Reps. Gabrielle Giffords and Ann Kirkpatrick have called for deploying the National Guard to the border, too.

There has been no corresponding call to increase the search for those who overstay visas.

In 2003, Immigration and Customs Enforcement created a special unit to track down visa violators. Funding grew from $6.7 million the first year to $68.3 million in fiscal year 2009, according to testimony in March by Assistant Homeland Security Secretary for ICE John Morton to the House Homeland Security Committee.Investigators analyze records of hundreds of thousands of potential violators based on data from various government databases that keep track of students, tourists and other people who enter the U.S. On average, the 272 investigators assigned to the unit arrest 1,400 visa violators a year, Morton said. ICE officials said the number of overstayers arrested each year has steadily risen, though they could not provide details.

Lon Weigand, assistant special agent in charge of ICE investigations in Phoenix, said most of the 27 visa violators in Arizona last year had overstayed tourist visas. ICE could not say whether that was up or down from previous years.

Border emphasis

A USA Today/Gallup poll released May 4 showed that four in 10 Americans think it is extremely important for the government to do more this year to control U.S. borders.

McCain, who sits on the Senate Homeland Security and Government Affairs Committee, said illegal immigrants who overstay their visas are a "serious problem." But "certainly not along the lines of what is happening on our border, though," McCain said, mentioning the killing of rancher Robert Krentz and the April 30 wounding of a Pinal County sheriff's deputy. Drug smugglers are suspected in both incidents. There also are fears that the increasing cartel violence in Mexico, where tens of thousands have been killed since the 2006 war on drugs began, could spill over the border.

"We've got drug smuggling. We've got murder. We've got abuse of human rights and human smugglers. We've got heavily armed and equipped organizations that are responsible for the deaths of 22,000 Mexican citizens," McCain said.

Cutler, the former immigration agent, likened the attention heaped on border security to a wing on an airplane. "Without the wing, the plane won't fly. But the wing alone does not constitute an airplane," he said.

Kirkpatrick, who sits on the House Homeland Security Committee, said the attention to the border is justified. "The risk of violence spilling into Arizona is clearly there. I am not going to wait for that risk to become a reality or for that problem to come to us," she said.

But she acknowledged that more attention needs to be paid to visa violators, especially keeping better track of people who don't leave after their visas expire. In general, people from other countries must obtain a visa to come to the United States as a student, tourist or businessperson. They are required to fill out an I-94 form when they arrive and turn it in when they leave. People from 36 "visa waiver" countries, including Britain, Spain and Ireland, are not required to obtain a visa if they plan to stay for less than 90 days for tourist or business purposes.

"The ease in which people can overstay their visas is a significant security threat," Kirkpatrick said.

Less focus on interior

The Pew Hispanic Center estimates that 40 to 50 percent of the nation's 11 million illegal immigrants entered the country legally. As of May 2006, the most current data available, 4 million to 5.5 million people had entered the U.S. legally and then remained after their visas had expired. An additional 250,000 to 500,000 people entered legally with temporary border-crossing cards and then stayed.

"Building fences and stopping people from sneaking into the country only goes part way," said Jeffrey Passel, senior demographer at the center who studies the undocumented population. Passel said 35 million people come to the U.S. with visas every year for extended stays. On top of that, there are millions of people who cross through land ports each year for short visits, making it difficult to keep track of everyone who enters and leaves.

"We allow this because it is good for the country. We want tourists to come here. We want students to come," Passel said. "But even if 99 percent turn around and leave," a significant number remain.

Gerald Burridge 58, an illegal immigrant from Britain, entered the U.S. on a three-year visitor's visa in 2003 and stayed after it expired. Interviewed at the ICE facility on Central Avenue in Phoenix, Burridge said he was arrested for failing to pay a drug-related fine and now is facing deportation.

Jena McNeill, a policy analyst for the Heritage Foundation, a conservative think tank, said that because most people overstay their visas to work illegally in the U.S., tracking those who enter and leave does little to solve the problem without also beefing up work-site enforcement.

"There are things we need to do at the border, but we need to get serious about interior enforcement," McNeill said.

Republic photographer Nick Oza contributed to this article.