Mexican drug lord Ismael “El Mayo” Zambada and the son of his former partner, Joaquin “El Chapo” Guzman, were arrested in El Paso, Texas, on Thursday, the U.S. Justice Department said.

“The Justice Department has taken into custody two additional alleged leaders of the Sinaloa Cartel, one of the most violent and powerful drug trafficking organizations in the world,” the department said in a statement.

Zambada and Joaquin Guzman Lopez, the son of El Chapo, face multiple charges in the U.S. “for heading the Cartel’s criminal operations, including its deadly fentanyl manufacturing and trafficking networks,” the statement said.

The arrest of Guzman Lopez was first reported by Reuters, ahead of the Justice Department statement.

Two U.S. officials told Reuters that Zambada and Guzman Lopez were detained after landing in a private plane.

Zambada is one of the most consequential traffickers in Mexico’s history and co-founded the Sinaloa Cartel with El Chapo, who was extradited to the United States in 2017 and is serving a life sentence in a maximum security prison.

After El Chapo’s extradition, his criminal empire was inherited by four of his sons — known as Los Chapitos, or Little Chapos — who took over his faction of the cartel and became some of the biggest exporters of fentanyl to the United States.

El Mayo and Los Chapitos have had a fractious relationship since El Chapo’s extradition, and the arrests of the two traffickers may trigger instability or even violence in Mexico.

U.S. federal prosecutors in February charged Zambada with conspiracy to manufacture and distribute fentanyl, which the Drug Enforcement Administration says is the leading cause of death for Americans between the ages of 18 and 45.

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