Five years ago, Mr. Valdez played a key role in the battle between the Sinaloa Cartel and the Gulf Cartel for control over the lucrative I-35 smuggling route into the United States, the Drug Enforcement Administration says.

He is also believed to be the person most responsible for pushing that conflict into central and southern Mexico, taking over the city of Acapulco.

Last week, Mr. Valdez was captured by dozens of federal police officers after a firefight at a rustic house in the mountains northwest of Mexico City. He had eluded the authorities for years despite having multimillion-dollar bounties on his head, and his capture was considered a major blow to the remnants of the Beltrán-Leyva organized crime group, law enforcement officials said.

For months, Mr. Valdez had been fighting for control of the gang since its leader and his mentor, Arturo Beltrán Leyva, was killed in a gun battle with the Mexican Marines last December in Cuernavaca, just south of the capital.

The internecine struggle had pitted Mr. Valdez against Mr. Beltrán Leyva’s brother, Hector. More than 150 people have died in the struggle, many of them mutilated or beheaded and left with grisly messages for the other side.

In videotaped statements to the Mexican police, Mr. Valdez said that he managed a smuggling route from Panama to Mexico and that he transported cash in tractor-trailers back from the United States.

He also admitted that he had ties to many of Mexico’s most wanted drug lords, including Joaquín Guzmán, who has emerged from the last three years of gangland warfare as the most powerful cartel leader in Mexico.

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Mr. Valdez faces three indictments in the United States. In Atlanta, where the most recent charges were filed, he stands accused of smuggling thousands of kilograms of cocaine from 2004 to 2006, shipping it through Laredo in tractor-trailers and then sending the millions in cash back the same way.

Kent A. Schaffer, a Houston lawyer representing Mr. Valdez, said his client denied all the charges against him. Mr. Schaffer also said Mr. Valdez was probably coerced into making the videotaped statements.

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Mr. Valdez got his start in crime as a petty marijuana dealer in Laredo in the early 1990s, but he was never arrested on drug charges, according to the Webb County Sheriff’s Office and the Laredo Police Department.

There were signs, however, that the affable linebacker on the United High School football team had a wild side: arrests for drunken driving and public drunkenness.

In 1992, near the end of his senior year, he was arrested on a charge of criminally negligent homicide after he drove his pickup down the wrong side of a road and collided head-on with a middle school guidance counselor, killing him. The charge was later dropped.

After graduation, Mr. Valdez turned down an offer from his father to attend college, saying he wanted to make money, his brother said. According to a federal indictment in Laredo, the next year he joined a group of smugglers who were moving hundreds of pounds of Mexican marijuana through Laredo to cities in Massachusetts and Missouri.

His brother said Mr. Valdez fled across the river into Nuevo Laredo in 1998 to avoid arrest, opened a small shop and never lived in the United States again. Detectives in Laredo say he quickly became affiliated with a local gang known as Los Chachos, one of four groups that controlled the city’s drug trade in those days.

Over the next years, the Gulf Cartel and its commandos, the Zetas, moved into Nuevo Laredo and started taking over the drug and extortion rackets from local gangs. Mr. Valdez was sucked up into the conflict.

Abel Valdez said his brother deeply resented the Zeta’s tactic of killing family members of their enemies and extorting enormous amounts from local businesses. Mr. Valdez joined in the fight against them partly in self-defense, and he has been battling them ever since, his brother said.

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“I’m not saying he’s an angel,” Abel Valdez said. “He’s done things that are not legally right, but he has principles.”

Los Chachos eventually lost control of the border town to the Zetas, who still control it, and Mr. Valdez threw his lot in with Arturo Beltrán Leyva, one of four major leaders in the Sinaloa Cartel, law enforcement officials said.

Mr. Valdez told the Mexican police this week that he first met Mr. Beltrán Leyva when he sought help in arranging a meeting with the head of the Gulf Cartel, Osiel Cardenas. Mr. Valdez wanted to plead with Mr. Cardenas to instruct the Zetas not to kill him.

By 2003, Mr. Valdez had been placed in charge of Mr. Beltrán Leyva’s squads of hitmen, known as Los Negros, law enforcement officials say. And a year later, he took over the gang’s operations in Acapulco, eventually pushing the Zetas of the Gulf Cartel out of the city with a bloody campaign that included beheadings and grenade attacks on police stations, Mexican officials said.

His rise was spectacularly rapid, and owed not only to his contacts in the United States, but to his brutality. Taking a page from Middle Eastern terrorists, he was willing to use beheadings, videos of killings and corpses to send messages.

“He seems to be a pretty bright kid and very brutal and ruthless,” said Scott Stewart, an analyst with the Stratfor security consulting firm in Austin, Tex., who tracks Mexican drug violence. “In a period of cartel warfare, the enforcers will tend to rise in the organization.”