"Every day, we have a seizure of some kind at this checkpoint," says Leslie Lawson, patrol agent in charge of the Border Patrol's Nogales Station. The border fence looms behind her.


NOGALES, Sonora - Alejandro Vega hiked five days through the Arizona desert and then toiled 10 years busing restaurant tables, building roads and cleaning manure out of horse corrals in the United States before his deportation in 2009.

Now, facing the southern side of a 20-foot-tall copper-hued fence in the border city of Nogales, Sonora, he says he's ready to risk prison or death to get back in.

"I don't care how many times I need to try," said Vega, 38, who in March scaled the barrier's iron slats and sprinted to a Walmart parking lot only to be caught and expelled again.

"My life is there - there's nothing for me in Mexico," he said. "Everything has its risk, but if you never risk, you never gain."

The daily struggle along the rugged Nogales frontier, which the U.S. government ranks as the highest-risk sector of its border with Mexico - a region where 120,000 people were caught crossing last year - points to a security challenge central to enactment of any new immigration law.

Senators are advancing a bill requiring that the Border Patrol show "90 percent effectiveness" in securing this and other high-risk border sectors - areas where more than 30,000 people a year are caught crossing - before legal rights are conferred on the estimated 11 million undocumented immigrants in the U.S.

The concern about border security, which Republican leaders call essential to a broader agreement on a path to citizenship for the undocumented, visas for guest-workers and farmworkers, and other elements of an immigration law rewrite, has only heightened following the April 15 Boston Marathon bombings. Two brothers whose family legally emigrated from Kyrgyzstan to the U.S. a decade ago and sought political asylum have been identified as the culprits.

"That's the No. 1 criterion," said Sen. Ron Johnson, R-Wis. "We want to treat the eventual problem with real humanity, but before that, we really do have to secure our border, not just because of the immigration issue but also just for national security."

In the House, where the immigration bill faces long odds, Judiciary Committee Chairman Bob Goodlatte, R-Va., calls border security "very crucial" to any plan - "exactly how it works in conjunction with the rest of immigration reform, it has yet to be decided," he said.

Legislation filed by a bipartisan group of eight senators demands a border-control plan with fencing and surveillance assuring that 90 percent of those who attempt to cross are caught or turned back to Mexico in these high-risk sectors before other steps are taken on immigration.

There are three such sectors: The area south of Tucson that includes Nogales; the border near Laredo, Texas; and the Rio Grande River valley near Brownsville, Texas. The effectiveness of security last year, according to a Government Accountability Office report based on Border Patrol data, has ranged from 87 percent in the Tucson Sector to 71 percent along the Rio Grande.

Senators say this makes the border-security in their plan obtainable, enabling the government to move forward with citizenship for the undocumented and other measures.

"The border-security triggers are strong but achievable," Sen. Charles Schumer, D-N.Y., who has visited the Arizona border in negotiations over the bill, said at an April 18 Washington news conference announcing it.

In the desert region south of Tucson that alternates between rocky gulches and 7,000-foot peaks, part of a 262-mile stretch of an almost 2,000-mile-long border, the challenge is spelled out in numbers: In this sector alone, 124,363 people were caught in 2011, the GAO reports. That's close to one-third of the 328,000 apprehensions along the entire Southwest border. Another 43,539 were turned back; an estimated 25,376 got away.

Manuel Padilla, chief patrol agent of the Tucson Sector, said calculating the effectiveness rate, which only applies in the border areas between ports of entry, is "not an exact science."

"In the urban areas, we have a very high effectiveness rate," he said. "Once you start getting into the rural environment, that's where it gets more difficult."

"Every day, we have a seizure of some kind at this checkpoint," said Leslie Lawson, patrol agent in charge of the Nogales Station.

In the desert surrounding the checkpoint, cameras and infrared scopes detect illicit movement. In the days after footprints and other evidence of illegal crossing are discovered, agents work to match the information with the immigrants they apprehend to determine their effectiveness rate.

Officials in Texas' Rio Grande Valley haven't had as much success in stemming illegal entries. While they've raised the sector's effectiveness rate from 55 percent in 2006, it remains the major area where migrants are most likely to successfully enter the U.S.

"It is doable," said Christopher Wilson, an associate with the Washington-based Woodrow Wilson Center's Mexico Institute. "When I first saw the 90 percent, that sounded really high to me, but the reality is, it is within reach."