One of the most notorious drug lords in Mexican history, the Osiel Cardenaswas released from a US prison on Friday and placed under immigration supervision, the US Federal Bureau of Prisons announced.

The former leader of the Gulf Cartel was responsible for some of the bloodiest, most violent acts in Mexico’s turbulent past. He has also been accused of ultra-violent tactics, such as beheading his opponents, in his attempt to control the drug trade.

Cardenas founded the “Zetas”, the armed wing of the Gulf Cartel, made up of former members of the military’s special forces. He was arrested after a fight in 2003 and extradited to the US in 2007. Three years later he was sentenced to 25 years in prison.

Later the Zetas began to operate on their own and for a time were the deadliest criminal group in Mexico, until they gradually disappeared.

A prison service spokesman told Reuters that Cardenas was turned over to Immigration and Customs Enforcement. It is unclear whether he will be deported to Mexico or remain in the US. A source close to the Mexican government said charges are pending against him in his country and he is currently in a US immigration detention center.

Leo Silva, a former US Drug Enforcement Agency (DEA) agent who worked in Mexico dealing with the Zetas, said Cardenas is directly responsible for horrific violent crimes that occurred in Mexico in the last two decades. The Zetas spearheaded efforts by organized crime groups to expand, from simple drug trafficking to extorting residents and businesses in the areas where they operated. The Zetas also spread terror by carrying out many kidnappings for ransom.

“This was something that Osiel created, which ushered in a new era of organized crime. He instigated this mentality of instilling fear in the country,” said Silva, who worked for the DEA in Mexico from 2008-15.