McALLEN, Texas -- Carlos Gonzalez gunned the throttle. The roar of the outboard motor warned everyone within a mile that the U.S. Border Patrol was rounding the serpentine bend of the Rio Grande known as the cola del diablo, or devil's tail.
Turtles scurried off logs and into the green, murky water as the Border Patrol boat raced by at 35 mph, flanked by a second boat with a hard-faced agent in dark sunglasses holding an M-4 carbine assault rifle.
On the Mexican side, men with fishing poles by their feet rested in the tropical sun. Their eyes followed every move the boats made. Gonzalez, a 20-year veteran of the Rio Grande beat, suspected that most were lookouts who would take out cell phones and alert smugglers when the coast was clear.
He cut off the motor and coasted into a foul-smelling landing less than half a football field across from Mexico, where countless feet had carved a well-worn path up the reedy riverbank. It was littered with inner tubes, plastic bags and underwear - unmistakable clues that immigrants continued crossing into Texas. But when the border agents went ashore, the beachhead was still. The only signs of life were swarms of buzzing mosquitoes.
"We're never going to stop them - never," Gonzales said. "This was happening before I was born, and it will be happening long after I am gone. There is no way to shut the river down."
Though politicians and regular folks all over America agree that stopping illegal immigration starts with sealing the border, agents on the front line are under no illusions that they can catch everyone - even with the 6,000 extra agents that President Bush plans to put on the border over the next two years.
The challenges of policing the southernmost boundary with Mexico were clearly visible recently as Gonzalez and two other agents fanned out on different shifts across the Rio Grande Valley sector, a stretch of 18 counties that ends where the river - feared for its murderous whirlpools and undertow - calmly empties into the Gulf of Mexico.
The 1,500 agents in this region are mainly small-town men from South Texas. They pride themselves on working hard each day to stop hardened criminals and terrorists, and they catch as many illegal immigrants as possible. But after years of experience on the Rio Grande, they realize that desperate people will always find a way across, as long as opportunities on one side remain so much better than on the other.
Equipped with all-terrain vehicles, mountain bikes, horses, helicopters, firearms, motion sensors and infrared cameras, the agents cast a sweeping dragnet over 17,000 square miles.
In addition to keeping vigil along the riverbank and staffing highway checkpoints to inspect cars and trucks heading north, agents board buses to question passengers riding to big cities in Texas. They double-check identification at regional airports. They stop freight trains and search cargo holds for immigrants riding the rails. They comb barrier islands for drug runners sneaking up the coast. And they scour the mesquite and brush of cattle ranches for smugglers cutting around the highway checkpoints.
The Rio Grande was one of the first places where the government got serious about fighting illegal immigration, and the Border Patrol says a steady buildup of agents and equipment has reduced drug running and human smuggling here over the past two decades. It has also improved the lives of hundreds of local men and women, who have found careers in the Border Patrol.
Many agents readily acknowledge that they did not join the Border Patrol because of a passion to stop illegal immigration. They were drawn to the job because it pays better than other lawful jobs in the region and promises a life of action along the untamable river Mexicans call Rio Bravo.
"I was looking for a career," said Derek Conrow, 29, an agent from El Paso who was Gonzalez's boat mate. "If you like the outdoors, this is the place to be. We're not stuck in the office all day."
Agents generally earn $51,972 a year after three years - twice the median household income in this region - and average about 25 percent more in overtime pay.
Gonzalez stood on the dock at Anzalduas County Park and looked out at another dock, in another park, in another country.
On weekends, each park fills up with Latino families picnicking as children wade in the river. When the agents aren't around, smugglers on Jet Skis scoot illegal immigrants from one side to the other in seconds, Gonzalez said.
He pushed off the dock and resumed his day patrol along the stretch of the Rio Grande southwest of McAllen, where the Border Patrol has a regional office. The 18-foot fiberglass boats are supposed to serve as deterrents. But they also make the agents, in white polo shirts with "Border Patrol" emblazoned on the back, highly visible targets.
Agents patrol the river in a state of unease, searching for an adversary who seems to be everywhere, yet nowhere. Smugglers have shot at them with semiautomatic weapons. Mexican peasants have pelted them with rocks. Someone tried to string thin metal wire across the river in an attempt to decapitate them. Agents have heard rumors that their names were on hit lists, compelling some to move.
The boats speeded up the river, past an old brick smokestack and the skeletal frames of cars scattered along the U.S. side. On the Mexican shore, a speedboat sat in a boathouse next to a Mediterranean-style mansion, and a white stallion grazed by the riverbank.
The agents slowed to a stop at a sandy peninsula they had nicknamed "the beach," where generations of illegal immigrants have forded the river and walked to havens in the neighborhoods nearby. To their surprise, they found a newly carved smugglers' cove. The fresh twine in the water told them it was being used to pull drug loads from one side to the other.
"Damn, I had never noticed that," Gonzalez said.
He had wanted to join the Border Patrol since he was a schoolboy in nearby Weslaco, where he would walk by El Bronco coffee shop on his way to class and see the agents in cowboy hats. It seemed like the best job in town, he said.
Gonzalez, 44, has not changed his mind about that, despite the dead bodies he has found floating in the water, the children he has seen drowning, or the humble-looking men he has caught who turned out to be convicted murderers and rapists. He is more convinced than ever that his job is important. But as one of those who protect the American border, he realizes that enforcement alone will not stop the flow.
If Hollywood dreamed up a Border Patrol agent, he would look like Pedro "Pete" Martinez, a tall, handsome and well-spoken 28-year veteran with the wilderness skills of an Eagle Scout.
Martinez peered into his binoculars and spotted a man crossing the river shortly before noon on a rubber tube in plain view. The man made it to the U.S. in a matter of seconds, then disappeared into a thicket of reeds. Martinez radioed for backup, and a pack of sport utility vehicles converged on the scene.
"You see anything? He's in there somewhere," Martinez said into his receiver.
In the Border Patrol's station in Harlingen, east of McAllen, inside a room full of monitors that resembled a television studio, an agent operating a sophisticated surveillance camera zoomed in on the spot where the man was last seen. Motion sensors scanned for suspicious vibrations in the surrounding fields of tall sugarcane and sorghum.
But the man was gone. He might not have been an illegal immigrant, Martinez said, because destitute Mexicans liked to scavenge in a dump nearby and swim home with mattresses on their backs.
"In this job, you have to learn what an alien is," the senior agent said. "They teach you that in the academy, but in the real world, you find it's different."
When Martinez spotted a footprint facing away from the river, he placed his index finger on it and gently pressed down. The dirt was dry and crusty, baked by the sun. That meant the mark was hours old, he said, so whoever left it was long gone.
Martinez, like others on the river beat, has developed sympathy for the desperate people he comes across.
"You deal with a lot of human misery - real human misery," he said. "You're dealing with the poorest of the poor. People are determined to get across the border no matter what. Jobs and the American dollar, that's the issue."
Agents caught 134,185 illegal immigrants from 71 countries in the Rio Grande Valley sector from October 2004 to September 2005, about half the number caught in 1997. Nearly 60 percent were from nations other than Mexico, with some hailing from as far as China.
Martinez, one of 12 children born to a carpenter from Brownsville, watched border agents work when he ran along the river levees as a teenager. Today, he coordinates the daily movements of dozens of agents.
In many ways the challenges have not changed over the years, said Martinez, 52. A constant problem is that some Mexican-American families along the border hide illegal immigrants out of pity or try to sneak in relatives.
"They have friends and family connections in Mexico, and they're trying to help somebody," said Martinez, a fourth-generation Texan whose grandmother lived in Matamoros, just across the river from Brownsville. "It's hard to deal with that."
Earlier in his shift, during a midmorning check at a Greyhound station, Martinez had watched as agents in khaki uniforms boarded a bus headed to Houston and began asking travelers for identification. Most passengers quickly handed over Texas driver's licenses and identification cards with respectful "yes, sir" answers.
Agent Rhaudell Cordova began questioning one nervous-looking man who said he was born in a Mexican village and moved legally to California as a child. Cordova asked him to name the hospital where he was born. It was a trick question; Cordova knew that in that part of Mexico, midwives delivered most babies. The man gave a satisfactory answer, and Cordova left him on the bus.
Cordova speaks Spanish well, but a few agents lack a strong command of the language, which they were taught during academy training. Martinez said about half are fluent.
"When you catch a smuggler and are trying to squeeze information out of him, trying to use reverse psychology and other interrogation techniques, it is important," Martinez said.
Martinez then dropped in on one of the least-exciting beats on the border: Valley International Airport. About 15 flights a day take off from the Harlingen airstrip to cities such as Austin and San Antonio.
Low-ranking agents stand around the terminal all day and watch for suspicious people. A few prefer the airport's air-conditioned comfort to the humid triple-digit heat of the river, but it is not a coveted assignment. Hours pass without any departures. The agents are lucky to catch one illegal immigrant a day.
"Even if we caught nobody, we'd be here," Martinez said. "If we left, the bad guys would notice."
Paul Backor said he was wasting away in a boring job as an office manager when he decided he wanted a more adventurous - and lucrative - career. Just before turning 37, the age limit for recruits at the time, he joined the Border Patrol.
Ten years later, Backor stared at a wide turn on the Rio Grande near Harlingen that agents had nicknamed "the lookout" and said he was happy with his decision.
As the sun began dropping in the distance, songbirds sang, and the twilight gave the water an ethereal glow. Soon the river would come alive with the sounds of smugglers and border crossers.
In 10 years, Backor will face mandatory retirement and will have to find a third career. A guarded man who made Clint Eastwood seem effusive, Backor, 47, did not philosophize about illegal immigration, or much else. He saw his job in simple terms.
"If they cross that river, they're breaking the law," he said. "When I go home at night, either I caught 'em or I didn't."
But when Backor catches mothers and fathers crossing with their children, he thinks about how much he wants to make life better for his daughters, Abby and Emily.
Backor drove to a stretch of railroad tracks just as agents with sniffing dogs were searching a train. A German shepherd named Bojar picked up a scent and dragged his handlers to an immigrant hiding in a metal freight car. The dog then rushed to a cubbyhole in another car. A second man stepped forward with an uneasy smile and turned himself in.
Agents cuffed the men, who had boarded hundreds of miles to the south in the Mexican state of Coahuila, pushed them in the back of a patrol wagon and drove off. The men would be returned to Mexico as soon as possible.
Back at the river, Backor stopped to chat with Rocky Bustinza, a young agent who grew up in Brownsville and joined the Border Patrol, just like his brother before him. Bustinza said he recently ran into one of his Porter High School classmates - in the booking room.
"I never would have suspected this guy was illegal," Bustinza said. But the man, who spoke flawless English, was caught crossing the river after visiting family in Mexico.
When darkness fell, it was absolute, save for the light of a half-moon, and Backor's patrol radio began to crackle with reports of immigrant sightings on infrared cameras and motion sensors. But this night, it seemed as if every report was followed by a call from an agent who rushed to the scene and found nothing.
Miguel Bustillo writes for the Los Angeles Times.