Editor's note: This story first appeared Sunday as an exclusive for our print readers.
Nobody disputes that's possible, but analysts and government officials say terrorists plotting to kill Americans are more likely to use other routes into the country, if they're not here already.
It's much more common for people convicted in the U.S. of crimes connected to international terrorism to have been U.S. citizens or legal residents, or come into the country on visas.
"There is no serious evidence that the U.S.-Mexico border is a significant threat from terrorism," said Edward Alden, a senior fellow at the Council on Foreign Relations, a nonpartisan think tank based in New York.
Claims of terrorist threats on the Southwest border distract legislators and policymakers from addressing long-term solutions to drug smuggling and illegal immigration, said Tom Barry, senior analyst at the Center for International Policy in Washington.
"It's politically motivated," Barry said, "playing on that sense of fear that certain people are susceptible to."
But proponents of tougher border enforcement say protecting Americans against terrorism motivates them, not politics.
"There's an enormous risk," said Michael Braun, who retired as chief of operations for the U.S. Drug Enforcement Administration in 2008. Members of Hezbollah, for example, "are absolute masters at identifying existing smuggling infrastructures on many borders around the world where they operate."
The State Department's 2009 "Country Reports on Terrorism" found that "no known international terrorist organizations had an operational presence in Mexico and no terrorist incidents targeting U.S. interests and personnel occurred on or originated from Mexican territory."
The State Department said that there was no evidence of ties between Mexican organized crime and international terrorist groups. But it warns: "The violence attributed to organized-crime groups on the border, however, continued to strain Mexico's law-enforcement capacities, creating potential vulnerabilities that terrorists seeking access to the United States could exploit."
Pinal County Sheriff Paul Babeu emphasized the risk of terrorists crossing the Mexican border into the U.S. in a May 26 open letter to President Obama.
"If the majority of regular illegal immigrants can sneak into America, what does this say about the ability of terrorist sleeper cells?" Babeu wrote. "The porous U.S.-Mexican border is the gravest national-security threat facing America."
Hiding in a car trunk
In his letter to the president, Babeu said thousands of illegal immigrants hailing from "special-interest countries" make the U.S.-Mexico border a national-security threat.
"In some cases, we have confirmed their troubling ties to terrorism," Babeu wrote. "Yet for those we apprehend, how many today live amongst us?"
The Border Patrol apprehended an average of 339 people from "special-interest countries" - those that warrant special handling based on terrorism risk factors - at the U.S.-Mexico border each year over the past six years, Homeland Security data show. That's less than 1 percent each year of the total apprehensions along the U.S.-Mexico border, Homeland Security figures show.
None of the 2,039 people arrested at the U.S.-Mexico border in that span presented a credible terrorist threat, Homeland Security officials say.
Homeland Security monitors, analyzes and gathers intelligence about potential threats but at this time "does not have any credible information on terrorist groups operating along the Southwest border," said department spokesman Matt Chandler.
Among the 36 people convicted by the U.S. Justice Department of charges relating to international terrorism last year, none came into the United States from Mexico. Half were U.S. citizens, most of them naturalized from countries such as Sudan or Somalia. Seven were extradited from other countries, while three were captured abroad by American forces. The others came to the United States on visas, or, in one case, were arrested while trying to come into the United States legally at a port of entry on the Canadian border.
However, over the last decade there have been a handful of cases in which suspected terrorists or Muslim extremists crossed the Mexican border illegally. None led to attempted terrorist attacks in the United States.
In 2001, for example, Mahmoud Youssef Kourani rode into the United States in the trunk of a car coming from Tijuana. He went on to live in Dearborn, Mich., but was arrested in 2003, and in 2005 was convicted of conspiring to send money to Hezbollah in Lebanon.
In January of this year, an extremist imam from Tunisia, Said Jaziri, was arrested while riding in the trunk of a car on a highway east of San Diego. Jaziri, who was expelled from Canada in 2007, hiked across the Mexican border with another illegal immigrant, and both were picked up by an American driver.
But a firefighter saw them pile into the trunk and reported them to Border Patrol agents, who pulled the car over and found the men. Jaziri was charged with a misdemeanor immigration violation.
not A traditional route
Over the last two decades, almost all of the known international terrorists arrested in the United States have come on legal visas or were allowed to come in without a visa, said Alden, of the Council on Foreign Relations.
"These are people that come on airplanes," said Alden, author of "The Closing of the American Border," which explains how the U.S. revised visa and border policies in the wake of the Sept. 11 terrorist attacks.
The 19 people involved in the Sept. 11 attacks entered the country on legal visas.
And over the last four to five years, the terrorist plots have increasingly involved people already in the United States - citizens and legal residents, he said.
"The notion of the (Southwest) border as the line that protects us from terrorism has really gone out of the window in the last several years," Alden said.
Not only is the U.S. side of the border heavily guarded, but the Mexican government makes an extraordinary effort to prevent terrorists from coming through its country. For instance, Mexico shares real-time information with the U.S. about airline passengers arriving in Mexico to make sure they don't include potential terrorists, Alden said.
The Mexican drug-smuggling organizations have no interest in allowing smuggling routes to be used by terrorist organizations either, he said.
"If it is discovered that a terrorist that carried out an attack in the United States came across the Mexican border, then the response would be further fortification of that border that shuts down smuggling routes and cuts into the profits," he said.
Being associated with terrorist groups would be very bad for business for drug-smuggling organizations, said Sylvia Longmire, a drug-war analyst and author. Proof of a terrorist coming through Mexico would have dire consequences for the Mexican government, too, she said.
But that point of view ignores the fact that terrorist groups and Latin American drug smugglers sometimes do business with each other and therefore have connections, said Braun, the former DEA operations chief, who now runs a security-consulting firm, Spectre Group International.
"Hezbollah is now heavily involved in the global cocaine trade," Braun said. "Most of the cocaine they're involved in distributing is heading toward Europe, but they're affiliating with the same cartels sending drugs to the United States."
That isn't to say the groups share an ideology, but simply that they have the connections needed to exploit smuggling routes into the United States. Also, people from the Middle East tend to have dark hair, dark eyes and olive skin, like most Latin Americans, so they can easily blend in, he said.
"On a moonless night at two in the morning, there's not a lot of due diligence going on when the coyotes and gatekeepers are moving human traffic across that border," Braun said.
Canada is a more likely crossing point because that country allows in more people as refugees and asylum seekers, said Henry Willis, a senior policy researcher on homeland security at the Rand Corp.
"To regard the Southwestern border as the 'frontline against terrorism,' as the Border Patrol does, is folly," wrote Barry, of the Center for International Policy, in a recent report.
People have talked about terrorists crossing the U.S.-Mexico border, but Anthony Coulson, who retired as head of the Drug Enforcement Administration's Tucson office last year, has seen hardly any evidence.
"Through the years I can probably count on my fingers on one hand the number of times that there was some type of terrorist activity associated with the border," Coulson said. "It just doesn't happen."
But that's not to say it couldn't.
"Border security at the southern U.S. border is a critical national-security issue," said Willis of the Rand Corp., "but for many reasons, not solely because of terrorism."