Mari Paz, who asked that her last name not be used, had left central Mexico bound for Houston, where she hoped for a joyful reunion with the son she hadn't seen in five years. Friends told the 50-year-old woman that the illegal journey wouldn't be all that difficult.
But nearly one month, half a dozen attempts and an injured knee later, a family reunion no longer figured in her plans; last month Mari Paz hobbled onto a plane to return home.
America's vast frontier with Mexico remains a highly porous landscape, where migrants by the hundreds of thousands cross annually. But stepped-up patrols, more barriers and high-tech monitoring have made the boundary impenetrable for many people.
Those who are turned back, like Mari Paz, are often less physically fit and middle-aged. They freeze from fright atop fences. They hurt themselves on nighttime journeys through gully-rutted terrain. They run too slowly to elude Border Patrol agents who spot them with remote cameras.
"This is where my dreams ended," said Mari Paz, at the border barrier in Tijuana. "Because of this fence, I haven't been able to see my son."
U.S. Border Patrol officials said the recent buildup has made it harder to cross and appears to be discouraging people from attempting it. From May 15, when President Bush announced the deployment of National Guard troops on the border, to July 23, the number of apprehensions for illegal crossings dropped 25 percent from the same period a year earlier.
"The perception, I believe, that is occurring in Mexico and, frankly, even further south of Mexico," said Border Patrol Chief David Aguilar, "is that our capabilities have increased dramatically."
Federal authorities do not have data showing how many people fail to make it across. But, in one of the few studies touching on the issue, researchers from the University of California, San Diego, found that as many as 8 percent of about 1,000 migrants in their study failed to cross.
Sara Hernandez, a 49-year-old from Guadalajara, said the fear of getting hurt would keep her from trying again to cross the two fences that separate Tijuana from San Diego.
"The first fence I jumped. But I never dreamed there would be another one, and that it would be so, so tall," said Hernandez, who fell from the top and sprained her ankle. She eventually went home.
Jorge Perez Diaz, 48, first came to the United States 25 years ago by walking along the beach from Tijuana. But when he tried crossing last year in the rugged hills east of San Diego, he was forced back by Border Patrol agents.
The fatigue he experienced was enough to discourage Perez from making another attempt to reunite with his wife in San Jose, Calif. "I'm too old now to walk across these mountains," he said.
Fearing long, brutal treks through the desert, people not in peak physical shape often head to urban areas. That's where they confront America's most fortified borders.
At the San Diego-Tijuana border, the two fences � the first 10 feet high and the second 15 feet high � line most of the frontier. Stadium lighting illuminates shadowy canyons. Motion sensors have been seeded across hills and beaches. Most recently, video surveillance cameras have been erected, and National Guard troops have arrived.
The number of apprehensions in the San Diego area jumped 18 percent in the period from Oct. 1 through Aug. 7 over the same period a year earlier.