Ismael "El Mayo" Zambada, Sinaloa cartel leader, arrested by US agents in El Paso, Texas.
Zambada, 76, co-founded the cartel with Joaquin "El Chapo" Guzman, now imprisoned in the US.
Guzman's son, Joaquin Guzman Lopez, was also arrested alongside Zambada, says the US Justice Department.
Zambada was charged with conspiracy to make and distribute fentanyl, fueling the US opioid crisis.
US Attorney General Merrick Garland called the cartel "one of the most violent and powerful."
Garland emphasized the Justice Department's commitment to holding cartel leaders accountable for fentanyl distribution.
The Sinaloa cartel is the largest drug supplier to the US, fueling the fentanyl crisis.
Fentanyl is the leading cause of death for Americans aged 18-45, authorities note.
The DEA offered a $15m reward for Zambada's capture, highlighting his significance.
Zambada also owned legitimate businesses in Mexico, including a milk company, bus line, and hotel.
Zambada’s nephew, Eliseo Imperial Castro, was killed in Mexico; he was also wanted by US authorities.
Zambada's arrest marks a major DEA victory against transnational criminal organizations, praised by the Biden administration.