Former Gulf Cartel leader Osiel Cárdenas Guillén leaves prison in the US

Mexican drug trafficker Osiel Cárdenas Guillén, former leader of the Gulf Cartel and founder of the bloodthirsty Los Zetas criminal gang, was released Friday from a U.S. jail and was taken into custody by Immigration and Customs Enforcement (ICE).

Cárdenas Guillén was released 21 years after being arrested and 17 years after being extradited to the United States, where he pleaded guilty to  drug trafficking, money laundering and extortion of U.S. federal agents.

"We confirmed that Osiel Cárdenas Guillén was released today," a source at the Federal Bureau of Prisons (BOP) told AFP.

"He passed into the custody of Immigration and Customs Enforcement," he added.

Cárdenas Guillén was released from Terre Haute Correctional Facility in Indiana, where he was serving his sentence, and "remains in ICE custody pending a final determination," an ICE spokesman later told the AFP.

In Mexico, he has several charges pending, but it is unknown whether the U.S. government will deport him.

The Gulf Cartel was once one of the most fearsome criminal groups in Mexico. However, in recent years it has lost influence and split into multiple factions.

As leader, Cárdenas Guillén, 57, oversaw a drug trafficking empire responsible for exporting thousands of kilos of cocaine and marijuana to the United States from Mexico, according to judicial sources.

Nicknamed "El Mata Amigos (the Friend Killer)," he was arrested in 2003 in Tamaulipas (northeastern Mexico) and extradited in 2007 to the United States, where he was sentenced in 2010 to 25 years in prison and ordered to forfeit $50 million.

The judge dismissed some drug trafficking charges, although he admitted to having hesitated to accept his plea bargain.

"I ask forgiveness from my country, Mexico, the United States, my wife and especially my children for all the mistakes I have made," Cárdenas Guillén said at the trial.

The gang boss used violence and intimidation as a means to further his goals.

Fight to the death

In 1999, the kingpin threatened to kill a sheriff's office agent working undercover with ICE after he refused to hand over a shipment of approximately 988 kilos of marijuana.

That same year, two agents - one from the U.S. Drug Enforcement Administration (DEA) and one from the U.S. Federal Bureau of Investigation (FBI) - were surrounded by Cárdenas Guillén and his gang and threatened with guns while driving in an official vehicle through Matamoros, Tamaulipas, in the line of duty.

The drug trafficker recruited former military personnel from the Mexican Special Forces to form his personal guard, but this protection force ended up operating on its own under the name of Los Zetas, one of Mexico's most bloodthirsty gangs until it dismantled.

After Cardenas Guillen's capture in 2003, The Zetas fought to the death with the Gulf Cartel for control of their territory and activities.

The Zetas, whose members usually dressed in black and used military-style ranks to differentiate themselves, such as "commanders," "veterans," "hawks" or "cobras," engaged in other activities in addition to drug trafficking, such as the trade of fuel stolen in Mexico to the United States and the kidnapping of migrants.