A Mexican drug kingpin who led a fearsome cartel for more than a decade pleaded guilty Friday to U.S. drug and racketeering charges.

Miguel Angel Caro Quintero pleaded guilty in Denver federal court to one count of racketeering in Colorado and one count of conspiracy to distribute marijuana in Arizona. He faces up to 20 years in prison when he is sentenced Feb. 4.

Caro Quintero, 46, led the Sonora Cartel, which U.S. authorities say smuggled thousands of tons of marijuana and cocaine to the United States in the 1980s. His brother, former cartel leader Rafael Caro Quintero, was convicted in the 1985 torture-slaying of undercover Drug Enforcement Administration agent Enrique "Kiki" Camarena Salazar.

Rafael Caro Quintero was arrested in Costa Rica in 1985 and extradited to Mexico, where he is serving a 40-year prison term for Camarena's murder.

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Authorities say Miguel Caro Quintero took up the drug operation after his brother's capture. In 2000, the brothers were identified as significant foreign narcotics traffickers under a federal law known as the Foreign Narcotics Kingpin Designation Act.

Caro Quintero seemed relaxed and smiled as he talked to his attorneys Friday. When U.S. District Judge Philip A. Brimmer asked if he understood the implications of his plea, he responded in Spanish: "I feel perfectly fine." He was led away in handcuffs.

"I know Mr. Quintero is very pleased to be on the road to getting closure on these cases," said his attorney, Walter Nash. In exchange for Caro Quintero's plea, prosecutors agreed to drop several other charges that would have sent him to prison for "many, many years," Nash said.

U.S. Attorney David Gaouette called it a "very significant case" and one of the largest drug distribution schemes ever in Colorado.

"I also think it showed the determination of law enforcement to pursue this, even though this occurred in the '80s," he said.

Prosecutors described an elaborate operation in which Caro Quintero exported two to four tons of marijuana a month to Colorado between 1985 and 1988. It was distributed throughout the United States by aircraft and vehicle.

Prosecutors estimate Caro Quintero may have supplied more than 100 tons of marijuana and received more $100 million over that period.

Prosecutors say Caro Quintero also conspired with two other people to import thousands of pounds of marijuana into Arizona and was recorded on the phone trying to sell marijuana to an undercover drug agent.

The Sonora Cartel's Colorado connection was uncovered in 1988, when a motel maid in Boulder found $42,000 in cash and reported it to authorities. The room's tenant said he was working for the cartel, and his arrest ultimately led authorities to seize about $10 million in cash, gold, artwork, rare coins and other valuables associated with cartel proceeds from storage lockers in Colorado and Arizona.

Caro Quintero was indicted in Colorado in 1990 on charges that included smuggling marijuana in half-ton amounts in 1987. The indictment alleged the gang dealt with large amounts of cash and that associates once carried $1.5 million in seven grocery bags in Boulder.

He was arrested in December 2001 in Los Mochis, Mexico, and served a drug sentence in that country. He was extradited to the United States in February. Since President Felipe Calderon took office in 2006, Mexico has extradited more than 190 drug suspects to the United States.