Osiel Cárdenas Guillén, a former Gulf Cartel leader and founder of Los Zetas criminal organization, was released from prison in the United States on Friday.

Cárdenas, 57, is a native of the northern border state of Tamaulipas. He was detained in Mexico in 2003 and extradited to the United States in 2007.

He reached an agreement with U.S. authorities and pleaded guilty in 2009 to the charges of drug trafficking, money laundering and making threats against U.S. federal agents. In 2010, he was sentenced to 25 years in prison.

Nicknamed “El Mata Amigos” (The Friend Killer), the Zetas founder was released early from the Terre Haute Federal Correctional Institute in Indiana for good behavior. The years he spent in jail in Mexico before his extradition to the United States were also taken into account.

The Drug Enforcement Administration (DEA) told the El Universal newspaper that Cárdenas won’t be sent back to Mexico, where there are valid warrants for his arrest. A DEA official told El Universal that he is free to go wherever he chooses.

However, other media outlets reported that Cárdenas could be handed over to Mexican authorities. Citing a Homeland Security Investigations (HSI) source, the Milenio newspaper said that he was turned over to U.S. Immigration and Customs Enforcement upon his release from prison, though the U.S. is not deporting him.

“Osiel will stay in the United States indefinitely or until his migratory situation is fixed,” the HSI source told Milenio.

The Matamoros native was the leader of the Gulf Cartel (CDG) at the time of his arrest, and considered one of the most powerful drug lords in Mexico.

During his 1997-2003 leadership of the cartel, “the CDG controlled a mammoth cocaine and marijuana trafficking empire that rivaled those of other storied Mexican organized crime groups, including the Sinaloa Cartel,” according to Insight Crime, a think tank and media organization that focuses on organized crime in the Americas.

While leader of the CDG, Cárdenas created Los Zetas, which served as the cartel’s armed enforcer wing until it struck out on its own in 2010. Los Zetas initially consisted of deserters from an elite unit of the Mexican army.

The group “professionalized Mexico’s gangland warfare by detonating an arms race and introducing a kind of brutal violence never before seen in the country,” Insight Crime reported Friday.

In 2010, Los Zetas murdered 72 migrants — 58 men and 14 women — in the municipality of San Fernando, Tamaulipas.

Michael Deibert, a journalist and author who wrote a book about the Gulf Cartel, told Insight Crime that Cárdenas is “arguably the most impactful, though not most famous, narco leader in Mexico.”

Mike Vigil, former head of international operations for the DEA, said that he was an “architect of extreme violence” and “his methods have become the blueprint for other cartels in Mexico.”

Insight Crime reported that Cárdenas “has no apparent remaining links to the CDG, which has fractured into smaller groups since his imprisonment.”

“However, the Cárdenas family remains a powerful force in Tamaulipas’ criminal arena,” it added.

With reports from El Universal, Milenio and Insight Crime