Alleged drug cartel boss Rafael Caro Quintero, who spent decades in prison in Mexico for the murder of a U.S. drug enforcement agent, pleaded not guilty in a U.S. court on Friday to drug trafficking charges that could result in his execution.

Mexico has sent drug lord Rafael Caro Quintero, who is wanted for the notorious killing of a U.S. DEA agent in 1985, to the United States with 28 other prisoners requested by the U.S. government, the Justice Department confirmed Thursday evening.

Rafael Caro Quintero, nicknamed “El Narco de Narcos” (The Narco of Narcos) is the founder of the now defunct Guadalajara Cartel—one of the most powerful drug cartels of the 1980s that later splintered into various groups, including the Sinaloa Cartel, one of the world’s most powerful criminal organizations and drug cartels.