San Diego — The mesquite bushes are thick and sharp. Agent Gabriel Arias weaves up through them, climbing the steep trail ahead of me. He pulls the black pistol out of his back left jeans pocket. Click-clack! He cocks it behind his back, then jams it into the pocket again. Further up, in a rocky clearing, the group of 11 men sees us coming. "Cinco!" Arias says into his walkie-talkie. It's the code for Leonardo Torres, his rear man in our group. "We're here." He moves up with his hands clear and empty, probably to indicate he is not an asaltapollo -- robber -- or La Migra from the United States.

Suddenly we're among them. "This reportero would like to ask you some questions," says Arias.

We're on the lower slopes of Tecate Peak. This is where illegal immigrants die. Either from the cold or from bandits. Last weekend it was the cold. At least nine people died and more than 140 were rescued from mountainous areas like this after the freezing weather hit.

Today (before the cold snap) we're more worried about smugglers and bandits. I have been climbing this mountainside with four Grupo Beta agents for 45 minutes. Grupo Beta is the Mexican migration department's task force, set up to fight banditry and violence on Mexico's side of the border zone. Tecate Peak -- or, as the Mexicans call the mountain, Cerro Cuchumá -- straddles the border, slopes down into a valley, and shelters Rancho La Puerta, the long-established spa for the rich and stressed.

We started off in the crisp, late-morning sunlight and drove up the lumpy road to the border. It was a regular patrol from Grupo Beta's headquarters on Calle 18, on Tecate's eastern outskirts. The patrol's job is to protect Mexican citizens crossing over into El Norte, even if their migration is illegal in U.S. eyes.

This is the badlands. The part migrants fear most. Where they're most vulnerable, where bandits roam the hills looking for pollos to pluck -- and rape, rob, beat, and murder. They know the migrants are easy pickings: They have money for their journey, maybe jewelry, and are desperate to make it through this last barrier.

We started walking westward, single file along the rusty border fence for a mile. Signs -- "Peligro: animales venenosos" ("Danger: poisonous animals") -- dot the fence, with pictures of scorpions, snakes, spiders.

And then...nothing. The fence just stopped. Beyond lies a horizon-full of native forest rising up to Cuchumá. Impossible to know where any border lay. Gabriel Arias, the man in charge of Grupo Beta Tecate, led us up through scrub and trees and scattered signs of migrants' expeditions. Blackened stone-circles where fires had burned. Biscuit boxes, one-gallon plastic water bottles tossed aside. Sun-faded yellow, blue, and white Squirt cans. Abandoned paint-spattered shoes. Toilet paper and scuffed earth, where people had defecated beside the trail. Even a rough shelter made with bushes, perhaps a family lost at night before making the crossing.

Down in the valley to the south: perfect-looking ranchitos and Rancho La Puerta itself, with people bending over, picking vegetables from their garden. Up to the right, the only sign of life on the American side is on the summit of Cuchumá, a tiny-looking hut with aerials bristling. "La Migra," says Torres. "Border Patrol."

"Cinco!" says his radio. It's Arias, further ahead. Torres listens, then suddenly says, "Down there!" He points away to a little road snaking along the valley floor. What looks like a black centipede winds slowly out of a bend and along the lane. It's a group of migrants making their way east. Now they cut north toward us and disappear under the slope. "Cinco!" spits the radio again. This time Torres points across the mountain. We spot another line of heads disappearing behind bushes. "I think that's the group that was attacked yesterday," he says. He hurries to get to a ridge, where Arias and the others are catching up with the second group. "One of that lot almost got killed yesterday," says Torres. "We were there. When we came across them, they were lying with their pants down being robbed by three gunmen."

"Show the reportero your shoulder," Arias says to the closest man in the group. Mario Alejandro Crisóstomo unbuttons the top of his shirt and exposes his left shoulder.

"Look," Arias says to me. "This is a lucky man."

An angry quarter-size scoop dents the flesh. "That's where the asaltapollo shot me with his .38 yesterday," says Alejandro.

The migrants laugh the laugh of survivors. But the man who seems to speak for the group, Ezequiel Rodríguez, a rapid-fire, clear-spoken 30-year-old man from Sinaloa, is serious. "Yesterday we were down there passing through Eagle Pass," he says. "We were resting, waiting for our guide. These three men came up. One had a cleaver, the other a .38 pistol, and the other an Uzi machine gun. They said, 'We're thieves. Lie down. Drop your trousers.' They took our jackets and shoes and watches and money -- everything we had. We asked them to leave enough to catch a bus back to Sinaloa. They refused. They are good for nothing! Assaulting their own people! Then they shot at Mario. Hit his shoulder. They would have killed him if these Beta people hadn't come up just then."

This was the miracle for Rodríguez's group. Grupo Beta appeared just as the robbers were collecting their loot. "We had been getting reports of these three," says agent Roberto Moreno. "We had been coming and waiting for them for days. Yesterday we were lucky."

The Beta agents persuaded the three men to give up without a shooting war. They took them in on suspicion of robbing and perhaps killing northbound migrants. "The great thing," says Moreno, "is we have lots of witnesses."

"We were lucky we have no women in our group," says Rodríguez. "They treat women and children badly. Sometimes they make women undress. Then they violate them." He says his group came from Tijuana on the Tecate bus. "We got off at Rancho La Puerta, then we came up here, and we're waiting for nighttime to cross. If the guide doesn't come we have to go back, because we'll get lost. That's what happened yesterday. We can't make it alone to America."

It's a strange place for an interview. The group lies sprawled on the rocks, under bushes. The Grupo Beta men catch their breath below. The breeze blows gently. Crows and hawks circle high above in the silence. But Ezequiel Rodríguez is suddenly hard to stop. "I'm coming north because where I'm from it is not a good life. No life! No money. I want a better future. To live an honest life. I have four sons in Sinaloa, a father and a mother and a wife to look after there. My dad said, 'What are you going to do, going north? They kill pollos like you -- they could kill you.' But what else can I do? I've got to try. If I get killed, oh well. That's what happens. It's a big decision, but you have to move forward, make things better. A visa is impossible unless a patrón writes a letter on your behalf.

"I'll work probably in the fields. I hear the pay is best in Washington and Utah and Oregon. I'm going to Washington to pick apples and pears. Apples go through May, June, July. It's hard work. I've done it before. In '97. We worked 14 or 15 hours a day. But the money is good. Back home if I'm lucky I make 30 pesos ($3) a day. The Migra say there are too many of us up there. But that's not true. In '97 I found work all the time. This time I'm looking to work maybe eight months and then return to my family in November-December.

"The first time I tried to come, in '86, it was the U.S. Border Patrol who attacked me. They ran me down. Trapped me. Hit me. Made my ear bleed. Guess the guy didn't like my attitude. Two of them kept hitting me. I asked them why. One said, 'Because you ran.' He hit me badly, but where it would not show. His blows bruised me internally. They left me for dead. But after they had gone, I was able to get up and crawl back to Tijuana. I went back down to Sinaloa, because they said if I stayed here they'd kill me. They hit me for nothing! We are just looking for a better life.

"That's why we're happy about Grupo Beta. We had never heard of them before. But first they save us from the asaltapollos yesterday, and then..." -- he brings out a folded 2I- by 3H-inch folded pamphlet -- "they give us this."

He hands it to me. Its cover reads: "Migrant: You Have Rights!"

Inside is a Mexican government-sponsored virtual Bill of Rights for the Illegal migrant. "IT IS PROHIBITED for 'La Migra [the U.S. Border Patrol] to use excessive force to detain you. Patrolling officers only have permission to use minimum force necessary.

"While holding you in custody 'La Migra' MUST NOT:

• aggress or insult you

• handcuff you too tightly

• neglect medical attention

• transport you in a dangerous way

• keep you in filthy or over-crowded cells

• deprive you of drinking water

• leave you without food for more than six hours, or take your money, jewelry, eyeglasses or medicines

"NOTE: 'La Migra' cannot separate minors from their parents who accompany them. If you have children they must give them food and warm covering without delay.

"IF 'LA MIGRA' MISTREATS YOU: Mexican consulates must intervene on your behalf. And you can also contact human rights groups.

"To give yourself a stronger case against patrolling officers, read their license plates and remember the numbers. At least note [the officers'] physical characteristics. Note the date, hour and place of the abuse. Note if there are any witnesses. And hold on to your [Border Patrol-issued] voluntary return papers.

"Keep this pamphlet, or give it to another migrant." Consulate and human rights groups' numbers are listed on the back.

"This is our job," says Arias. "To protect migrants from abuse. That's what Grupo Beta is for." The force started in late 1991 when border banditry was getting out of hand. San Diego's police had sent undercover patrols to the border. Shoot-outs were common. The patrols became suddenly famous when Joseph Wambaugh turned their exploits into his book Lines and Shadows.

Mexico's response was Grupo Beta, an elite independent group of Mexican law enforcement officers drawn from municipal and state police and the Mexican immigration service. They were paid their regular salaries plus an additional federal wage. That made them the highest-paid cops in Baja. The job of the original 35 was as it is now: to wear plain clothes, blend in with migrants and face down bandits on their own turf. Grupo Beta agents became known for their honesty and independence. They were given the moniker "Mexico's Untouchables." There have been some recent voices saying Grupo Beta has seen better days. According to American media, such as the Union Tribune, illegal immigrants deported back into Mexico have told tales of being abused and shaken down by Beta agents, instead of being protected by them. But agent Roberto Moreno, one of the original Tecate members, says the past year has seen a weeding out of bad agents, better training, and more attention and equipment supplied from Mexico City. "We are back to our original mission," he says.

Torres says today was easy. Patrolling these same trails at night, when most polleros -- guides -- take their "chickens" over the mountain and into America, is something else again. You don't get four-wheel rides through this country.

I ask Torres why he does it. "I'm protecting our citizens and the law," he says. "Somebody has to stop these bad people." The sun is slanting west of Cerro Cuchumá. "We're going to eat," says Rodríguez. "We still have some water and food the robbers didn't take. Sardines, frijoles, chiles. Then we'll wait for our guide. This is where we're supposed to meet. He'll come before nightfall. We paid $700 for him, in Sinaloa, before we left. About $70 each. We know he is good and responsible."

But what if the guide doesn't come? "We can't go back," says Rodríguez. "We have no money to go home. We'll just have to try again. But we are tercos. We're stubborn." On the way down, we come across a small clearing with a big wooden cross. "We had a shoot-out with bandits here," says Leonardo Torres. "The brother of Eduardo Alvarado -- he's the agent up ahead with Mr. Arias -- was killed. Carlos Alvarado. May 17, 1996. You wouldn't want to talk to Eduardo about it yet. He still feels it too strongly."