Cesar spoke to CNN many times about his life and what he's done. This is his account, verified where possible through public records and interviews with other figures in his life. As in the accompanying video, we have changed names or used middle names to protect the identities of the young men and their families still at risk from cartel violence.

Eagle Pass, Texas (CNN) Cesar was 15 when he thought that, finally, he had turned his life around.

Just 15, but he already had much he wanted to distance himself from: drug trafficking, human smuggling, murder. He even blamed himself for the death of his toddler brother in a tragic accident outside their home in Texas.

But now he was staying out of trouble, going to school, working with his dad on construction sites.

And then he got a phone call that reminded him his past was very much still his present.

It was September 25, 2013. A Wednesday. At 8:05 p.m.

He texted his girlfriend to say goodbye. He updated his Facebook status in a mix of Spanish and English to say his life was over.

"It all had to end like this. Yo sabia que tarde o temprano, me iba tronar. Dios, cuida a mi familia." I knew that late or early, I would die. God, take care of my family.

He sent a text to his probation officer: "I have a feeling I will not make 16 don't tell my mom I'm telling you this whenever I die let her know that I always loved her."

The phone call had been from a drug cartel telling him he was needed for a job. We are coming for you. No need to hide.

Cesar didn't want to do their business any more. He thought he'd gotten out of gang life. But the cartel had been explicit: Cesar would pay if they didn't get what they wanted.

Despite the threats, he was steadfast. He was done with it. He wanted to rest. So he made peace with what was coming; he thought he would die that night.

"Straight up this is the end of my story," he texted his probation officer.

This is where it stops. This is where it ends.

His probation officer called the sheriff, and officers were sent to his house. They found Cesar armed with two kitchen knives to defend himself against his killers. Under his bed they found a safe and a bulletproof vest.

Cesar was taken into custody and given a psychological evaluation. At first he wouldn't talk. And then he revealed the death threats against him and a little of what he'd been through. He was diagnosed with a nervous breakdown. At the age of 15.

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Cesar was brought to Eagle Pass, Texas, from Mexico by his parents when he was 3 years old. They were undocumented but his father, Juan, was able to find enough construction jobs off the books to support his family, even in Eagle Pass, where poverty and dirt roads outnumber jobs and opportunities. He and his wife, Ester, a devout Christian, provided a stable, loving home for Cesar and his three siblings who would be born later.

Cesar's mother holds rosaries he made for her.

Cesar enjoyed school and loved sports. He played on the football and baseball teams, collecting trophies, until junior high, when his own undocumented status meant he couldn't go to out-of-town games.

Cesar's house is about a mile from the Mexican border and Cesar and the neighborhood children often scrambled up the hills to look from Eagle Pass across the Rio Grande to Piedras Negras on the Mexico side.

The two towns now linked by bridges for vehicles and pedestrians each have a colorful history. Piedras Negras claims to be "where the nacho was invented" while a slogan for Eagle Pass says it's "Where Yee-haw meets Olé." On the American side, bus companies advertise rides to San Antonio, the nearest big city, about 140 miles away, while Border Burger touts the "best burger and taco special in town."

Mexican and American influences abound in this border town in the language, cuisine and culture of its 28,000 or so residents. But so does the dark side of the modern border -- the smuggling of drugs and people. It's all around, waiting to impact, perhaps destroy lives, even that of a boy.

Sometimes the gang life emerges on those overlooks, where one of the children will say to another, would you like to make some easy money?

For Cesar, it was a direct introduction from a boy he thought was his friend.

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Leo was a little older than Cesar but lived in the same neighborhood and they started to hang out. He always seemed to have money.

The summer after sixth grade, 12-year-old Cesar was at a party with Leo when he was introduced to another boy, Alejandro.

Two days later, Leo called Cesar to pass on a message from Alejandro: Do you want to work for Los Zetas?

Cesar said no, even when Alejandro came by to ask him in person.

But a couple of days later, Alejandro called Cesar and told him to go with Leo on a job for Los Zetas, then and now one of Mexico's most violent, vicious and powerful gangs.

He said Cesar would be sorry if he didn't go.

So Cesar went.

Leo and Cesar drove a truck away from town down El Indio Highway and pulled off to the side.

Eight or so masked men ran from the surrounding fields to the vehicle, opening the door and throwing in eight sacks of marijuana.

The door was closed and the boys drove back to Eagle Pass. Not a word had been spoken.

The border fence between Eagle Pass and Piedras Negras winds up and down the hills.

They dropped off the truck with Alejandro, were given a ride to the mall and a down payment of $1,000 in cash with $6,000 more to come.

That scenario played out again and again, with Alejandro assigning jobs to Cesar and Leo. Cesar said he always told Alejandro he wanted no part of it but Alejandro's reply was that Los Zetas were impressed by him and that more senior cartel members wanted to meet him.

And he kept going along.

One day, outside Eagle Pass Junior High School, Cesar got in a truck to be told that after one more job, he would belong to Los Zetas forever.

Even as he said he protested, they gave him a cell phone and told him they would call soon.

He refused to answer the calls, and then a man came to his house, saying he was the boy's commander.

"You work for me. You do what I say when I say it and how I say to do it. Without any errors or I will kill you."

The "work" started that same night, with Cesar ordered to drive a truck 20 miles to a ranch where it was stripped, packed with cash and reassembled for Cesar to drive back to Eagle Pass.

For that he was paid $15,000 in cash, handed to him through a car window by someone he never saw.

He hadn't even started seventh grade.

There was no way out.

Perhaps Cesar wasn't looking for that way out, for he was also beginning to enjoy the trappings of gang life. In return for smuggling drugs and moving people he got pay and perks.

"Money, protection, parties," he said later -- those were the good things. He went to parties in clubs and at ranches in San Antonio and Houston, and across the border in Puebla, Guanajuato, Monterrey and Sinaloa. If anyone truly bothered him, he knew he could make a call and have them warned, scared or worse.

Federal police patrol the streets of Piedras Negras.

"Sometimes it makes me feel proud of myself," Cesar said. "Because not any [teenager] can say 'Oh, if you mess with me, I can kill you.' Not me but I can send people to kill you and your whole family."

He certainly didn't tell his parents; he was too embarrassed and knew they would disapprove. Juan and Ester were doing their best to bring their American dream to their children. They worked hard to maintain their blue-collar lifestyle without seeking any government assistance and unlike many on their block, they didn't resort to drugs.

So Cesar hid his cartel life, walling that part of himself off, even as he was beginning to feel he deserved it. "I chose this life. Now I have to live it ... face the consequences. Anything that happens."

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There were other boys in Eagle Pass working for Los Zetas, including Eduardo, who was about 15 and had been smuggled from Mexico to work for the gang.

One day Cesar, Leo, Eduardo and some other youths were ordered to go to Mexico. Los Zetas had a favored route across the Rio Grande where the river was nearly dry and you could cross the border unmolested without even getting your feet wet. And they also had a number of Border Patrol agents in their pocket, paid to look away if teens were seen crossing the river, Cesar said.

The boys had been told there would be a party but when they arrived, armed men rushed them, hitting Eduardo in the back of the head and throwing him on a truck.

The other teens were loaded on another vehicle and taken to a ranch where they were told to line up.

All except Eduardo.

"This is what happens if you don't do what we say," the boss said as Eduardo was pulled off the truck and forced to kneel.

The boss pushed Eduardo's head back and cut his throat with a "huge knife," Cesar said.

Then another man took Eduardo's body to one side and chopped off his head with an ax.

Cesar said he'd been told Eduardo had been called on to do a job but replied that he couldn't do it at that exact moment.

Because of that, Eduardo was killed.

And from then on, Cesar did exactly what he was told.

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His main role was to pick up vehicles packed with cocaine and marijuana that had been smuggled into the United States and drive them with other youths three hours away to San Antonio.

Completing job after job without getting caught won him credibility, and more work.

A video camera watches over a landmark commonly used as a rendezvous by people crossing the border.

His boss came to trust him and introduced him to more senior cartel members, once at a ranch on the Mexico side of the border.

Cesar said he was scared but one comandante told him he had "heard good things" about him and he and other chiefs gave Cesar their cellphone numbers, telling him to call if ever he needed to.

Before he left the ranch, a man accused of stealing $325,000 was brought in. When he denied the theft, the commander became angry, pulled out his gun and shot the man three or four times in the head.

It's possible Cesar, now 13, was becoming inured to the life and death around him. But then death came to his home.

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It was May 12, 2011, and Cesar was out of school, suspended for three days for talking back to a teacher.

The silver lining was that he could spend the day with his beloved 2-year-old brother, swimming and watching TV.

Their mom checked in on them after work before heading to a Zumba class, returning about 9:30 p.m.

The toddler heard his mom's voice and ran outside. At the same moment a neighbor was pulling her truck into the driveway.

He was struck and died later in the hospital, two months before his third birthday.

No one blamed Cesar - except Cesar himself, who thought if only he hadn't left the gate open.

He couldn't move on. Now he felt there was nothing good in his life, not one redeeming factor. He thought he was no better than the cartel and that should be his life. But with the complex emotions of adolescence, he also began to chafe against the cartel authorities.

That same year, Cesar's chief was replaced. His new boss was violent and explosive, killing for any reason, even once burning a bus with children and adults inside it, Cesar said.

Cesar was no longer a favored son and the pressure increased.

"I told him that I didn't want to work for Los Zetas anymore and that I would rather be killed than to continue to work under him," Cesar said he declared after one night of constant text orders.

The new boss called at 5 a.m. and told Cesar he was coming to pick him up to kill him.

They met outside Cesar's house. The man pointed a gun at Cesar's face.

Then, he started laughing.

He told Cesar he wasn't going to kill him. But that he would keep working or his family would be killed. He had been working with the cartel for too long. He would have to work again that morning.

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That job was moving marijuana again. But this time, the Border Patrol was waiting and Cesar was arrested for the first time - with 240 pounds of marijuana, then worth an estimated $192,000.

Like most children arrested for drug trafficking, Cesar was taken to a detention center. He was released 10 days later and was assigned a probation officer.

Teens walk in line at a detention center.

Later, Cesar said one of the comandantes he'd met in Mexico called and asked him what happened.

"I told him everything -- how I had told [my chief] that I didn't want to work anymore and how he had come to my house, pointed a gun to my face, and forced me to work saying that he would kill my family if I didn't. [The comandante] told me that I could leave Los Zetas now and that he would take care of my boss. And that was it for me with Los Zetas. I took the opportunity to leave as soon as it was given to me."

Cesar said he had won favor and the right to get out because he'd been so successful for so long.

In July 2011, about a year after his first Zetas job, the cartel discharged him.

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For two years, he started to rebuild his life. Working at school, working with his dad, staying away from the cartels.

Until that phone call from a rival gang, Cartel del Golfo, which was taking over territory from Los Zetas.

They'd been trying to recruit him for a few weeks, saying how much they liked his work for Los Zetas and how valued he would be, especially since he could give them inside information on their rival.

And if the flattery didn't work, maybe the threats would.

Cesar knew his age would be no reason he would be treated kindly. He wouldn't even be the first 15-year-old to disappear from Eagle Pass and turn up dead. Officials here can rattle off three other cases of boys dead or missing in recent months, with suspicions falling on the cartels.

But still, Cesar couldn't say yes, so he prepared to be killed.

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The actions of the probation officer, the sheriff's deputies and the medical staff saved Cesar that night and in the days after.

For them, he's one of many. The probation officer says he and his colleagues have become involved with about 100 children used as drug couriers for the Zetas since Cesar was arrested four years ago. Others likely go undetected, as Cesar did for so long.

In Eagle Pass, Texas, where poverty and dirt roads outnumber jobs and opportunities, Mexico's drug cartels prey on kids --- offering them thousands of dollars to smuggle hundreds of pounds of drugs each week.

But the probation officer also sees a special chance for Cesar, even now as he sits as a 17-year-old in a detention center awaiting a deportation hearing triggered after he was taken into custody for a violation while he was driving. That official is supporting Cesar's family's efforts to gain legal status.

Cesar still has the support of both parents and his siblings. Doctors say his mental and physical scars can heal with time and treatment. And he wants to get better, to enjoy life and to contribute.

He wants to regain the sense of purpose and optimism he had. When he was 15.