MEXICO CITY (AP) — Osiel Cárdenas Guillén, one of Mexico's most-feared drug lords, has been released from a U.S. prison after serving most of a 25-year prison sentence, authorities confirmed Friday.

A U.S. Bureau of Prisons official said Cárdenas Guillén had been released from prison and was placed in the custody of the U.S. Immigration and Customs Enforcement. That would normally suggest he would be deported back to Mexico.

A Mexican official who was not authorized to be quoted by name said Cárdenas Guillén faces two arrest warrants in Mexico, making it likely he would be detained upon arrival.

The former head of the Gulf cartel was known for his brutality. He created the most bloodthirsty gang of hitmen Mexico has ever known, the Zetas, which routinely slaughtered migrants and innocent people.

Cárdenas Guillén was sentenced to 25 years in prison in 2010 and ordered to forfeit tens of millions of dollars. It was not clear why he did not serve his full sentence, but he had been extradited to the U.S. in January 2007.

The 57-year-old native of the border city of Matamoros, Mexico, moved tons of cocaine and made millions of dollars through the Gulf cartel, based in the border cities of Reynosa and Matamoros.

He created the Zetas, a gang of former Mexican special forces soldiers who he recruited to become his private army and hit squad. They committed acts of terror that regularly involved slaughtering dozens of people, decapitating them or dumping heaps of hacked-up bodies on roadways.

The Zetas lived on long after Cárdenas Guillén was captured in 2003. By 2010, the Zetas had formed their own cartel, spreading terror-style attacks across Mexico as far south as Tabasco until their top leaders were killed or arrested in 2012-2013.

An offshoot of the Zetas, the Northeast cartel, continues to control the border city of Nuevo Laredo, across from Laredo, Texas.

But Cárdenas Guillén's own gang, the Gulf cartel, has become hopelessly splintered after more than a decade of bloody infighting between factions with names like The Metros, The Cyclones, The Reds and The Scorpions.

Cárdenas Guillén's own nickname was "El Mata Amigos," or "The one who kills his friends."

Cárdenas Guillén's most brazen act was when he surrounded and stopped a vehicle carrying two U.S. Drug Enforcement Administration agents and one of their informants in 1999 in the border city of Matamoros, across from Brownsville, Texas.

His gunmen pointed their weapons at the agents and demanded they hand over the informant, who would almost certainly be tortured and killed. The agents toughed it out and refused, reminding him it would be a bad decision to kill employees of the DEA. Cárdenas Guillén eventually called off his gunmen, but not before reportedly saying "You gringos, this is my territory."

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Alanna Durkin Richer contributed from Washington, D.C.

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