LA GRULLA, Tex. — Manuel Torresmutt, a Border Patrol agent, pulls his white and green Chevy Tahoe to the side of a deserted gravel road, framed on one side by railroad tracks and on the other by thick green brush.
The South Texas sun streams brightly as Mr. Torresmutt, a stocky, 24-year veteran of the United States Border Patrol, steps from his truck to meet with his three-man team. A radio dispatcher says “four bodies” have been spotted on the “Mike” side, referring to the code name for the bank of the Rio Grande in Mexico.
A few minutes later, Mr. Torresmutt and other members of the Border Patrol’s Rio Grande Valley Sector Horse Unit are on their way, rocks and dust flying as their mustangs rush into the bush in search of those crossing the border illegally.
President Trump has promised to build a wall to stop the flow of illegal immigrants in areas like this, but the geography — like shallow riverbanks and craggy trails that are impassable for vehicles — makes that nearly impossible and staggeringly expensive.
Since the Sept. 11, 2001, attacks, the United States has spent over $100 billion on various border security technology, including ground sensors, video cameras, walls, layers of fencing and infrared cameras. But in the thicket along the river where smugglers can easily hide, the horse patrol unit plays an essential role in efforts to detect illegal activity.
Just over two years ago, asylum seekers fleeing persecution in Central America swarmed the Rio Grande Valley, and many of them eagerly surrendered to Border Patrol agents.
But in the past few months, the number of immigrants caught illegally crossing here has plummeted to about 100 people a day from over 600. John F. Kelly, the homeland security secretary, has said the drop stems from aggressive enforcement, but migrant experts say improving conditions in the countries that people have been fleeing have also contributed to the decline.
Border Patrol agents comb the area in trucks and on bicycles on main roads. Helicopters and blimps, known as aerostats, provide aerial surveillance, while boats keep watch over the waterways.
The horse patrol unit works close to the Rio Grande, where agents say a border crosser can slip through wooded areas and in a few minutes be in town or spirited away in a vehicle.
While the flow of migrants has slowed, drug smuggling remains constant. The signs are everywhere: Dozens of footprints, abandoned life jackets, swimming trunks and food wrappers appear along the riverbank.
The Border Patrol unit here has almost 40 horses, obtained from a wild mustang program run by the federal Bureau of Land Management. The agency catches horses and turns them over for basic training to inmates at the Kansas City Correctional Facility.
Horses have been used for border security since the inception of the Border Patrol in 1924. The horse patrol unit here was established in 2011, after the local chief of the Border Patrol realized that the animals were well suited for hard-to-patrol areas. Their keen sight and hearing are also useful, said Jeff Wiggins, an agent who oversees the training of horses.
A little over an hour after riding off into the bush, Mr. Torresmutt and another agent, David Garcia, return to the staging area.
Two other agents, Garrett Gremes and Kelby Forbes, are chasing a man who has made it past both the horse patrol and agents in vehicles.
As evening sets in and mosquitoes buzz, radio chatter signals that agents and local law enforcement officers have spotted drugs dumped on the Mexican side of the border.
While Mr. Torresmutt and his unit decide their next move, radio dispatchers report that agents have made dozens of roadside apprehensions.
A short time later, a dispatcher alerts the unit to a group of people who are crossing the river in a small raft. That information is being relayed from radar aboard an aerostat flying in the area.
Within an hour, an alert is sent to Mr. Forbes’s cellphone from a remotely operated camera system, known as Operation Drawbridge. The agent clicks on a link and is instantly given the coordinates to the areas where a camera has spotted the group.
Operation Drawbridge, which is run by the Texas Department of Public Safety, is a network of thousands of hidden wildlife cameras equipped with motion detection and lowlight capabilities. State monitors receive an alert each time a camera detects activity and provide agents in the field with round-the-clock images of possible illegal crossings.
Mr. Torresmutt gathers his unit to devise a strategy to catch the crossers. He and Mr. Forbes will approach from the east, while two other agents will come from the north.
“We good?” Mr. Torresmutt asks. Everyone nods in agreement. The agents gallop toward the river.
It is nearly midnight when a Border Patrol van pulls up.
Three men are in the back. They were captured by Mr. Torresmutt and the horseback unit while trying to elude agents by running back into the river. The men had been turned over to another agent driving the van. All are wet and covered in mud.
One says he is a Mexican citizen. The two others say they are from Guatemala and have been traveling for over a month.
The agents don’t believe them. The three men have cellphones and seemed to be communicating with people on the Mexican side of the river. The agents suspect they are drug runners or scouts for the cartels.
“How much did you pay to get here?” Roderick Kise, a public relations officer for United States Customs and Border Protection, asks the men in Spanish.
The three men shake their heads in unison, indicating that they had not paid anyone.
“Por favor!” Mr. Kise says in disbelief. He asks again whom they paid for passage.
The three men stand firm.
“Por favor!” Mr. Kise says again, raising his voice. “You cannot cross from Mexico without one of the cartels. Nothing is free, especially over on that side.”
Mr. Torresmutt and the horseback unit arrive just as the interview of the captured men is wrapping up.
Both the agents and the horses are covered in sweat. Mr. Forbes’s horse stepped on a rattlesnake during the chase, he says.
The three apprehended men are part of a larger group, the agents say. Several members of the group fled back into the river after spotting the horse patrol. Agents did not pursue them into the river, but stayed and signaled to possible smugglers on the Mexican side to come and get the men.
At least one man managed to evade capture, according to the agents. He most likely made it into town and disappeared. Finding him is a task for the next shift.
Mr. Torresmutt and the team load the horses back into a trailer. After 10 hours, their shift is over. They will head back to the stables, where the horses will be rubbed down and watered. The agents will file their paperwork and then head home.
“Tomorrow we’re back at it,” Mr. Torresmutt said.
©, 2017, The New York Times