Dan Slater was in New York, freshly laid off from the Wall Street Journal and wondering what was next for him. On a June morning in 2009, a headline in the New York Times caught his eye: "Drug Cartels in Mexico Lure American Teenagers as Killers."

His attention piqued, Slater read the story. Then he read it again. And again. He soaked up the details of Gabriel Cardona and Rosalio "Bart" Reta, who were recruited as assassins for the Zetas drug cartel when they were still teens. The details haunted him. How did two kids from Laredo, Texas, become hired killers for a fearsome narcotics ring across the border? He couldn't stop thinking about it.

From such obsessions books are born. After seven years and thousands of pages of correspondence, we have "Wolf Boys," Slater's evenhanded, exhaustively reported and frighteningly intimate snapshot of a dark, bloody corner of the drug trade.

As Slater recounted over coffee, he had a lot of questions: "Where do these kids come from? Is this a thing? I knew very little, next to nothing, about Laredo. I really didn't know what it all added up to at the time. It was just one of those stories that stuck in my head."

It stuck there as he visited Cardona and Reta, both doing life in prison. It stuck as they exchanged letters, and as Slater began getting some answers to his questions. Reta, who was 13 when the Zetas recruited him, eventually cut off correspondence; he felt he'd been burned by other media accounts. But Cardona kept writing back. A stone-cold killer of ambiguous remorse, he held little back.

"He's a very sensitive guy and a very detail-oriented guy," said Slater, a 39-year-old Minneapolis native. "In a weird way, being in solitary confinement sharpened his senses a lot. He's very attuned to the physical feel of a place."

Slater isn't interested in making excuses for Cardona. He knows his subject is a murderer. He also knows, as a writer, that Cardona is a rich character, a popular, handsome kid who decided that killing people would be a good way to make some money and live a life of plenty.

And Slater knew that an anti-drug lecture would make for a pretty boring read.

The way he looked at it, "These guys have been written off as monsters already. They're animals in a cage at this point. What am I going to say condemnation-wise, scolding?"

It helped to find and cultivate a compelling secondary character on the other side of the law. Enter Robert Garcia, a Mexican immigrant, law enforcement lifer and veteran soldier in the drug war, for the DEA and the Laredo Police Department.

In the book, Garcia keeps arresting Cardona, and keeps watching him walk out on bail. He helps set up the sting operation that brings Cardona in for good. But he grows more and more skeptical of the drug war. As Slater writes, "Robert's arc of lost innocence, from fervent drug warrior to disillusioned critic, seemed to contain the entire story of American drug prohibition."

There's a bit of irony at the core of "Wolf Boys." At a time when anti-immigration noise is at fever pitch, this is the story of Americans who cross into Mexico to kill people, and the Mexican-born detective who brings them to justice in the States.

Not that any of that drove Cardona to kill. He merely saw an opportunity to advance, and he took it. He's a reminder of how thin the line between American dream and American nightmare can get.