The U.S. Attorney's Office in Chicago says a high-ranking member of the Sinaloa cartel in Mexico pleaded guilty to drug trafficking a year ago and has been cooperating with authorities.

Documents unsealed Thursday show Jesus Vicente Zambada Niebla pleaded guilty in April 2013. A co-defendant is Joaquin Guzman Loera, also known as "El Chapo," who was arrested in February.

Zambada was extradited to the U.S. in 2010. He pleaded guilty to one count of conspiracy to possess with intent to distribute multiple kilograms of cocaine and heroin between 2005 and 2008.

Authorities say he is responsible for distributing tons of cocaine in the U.S.

Court documents show Zambada admitted he was "trusted lieutenant" for his father, Ismael Zambada Garcia, considered to be co-leader of the cartel with Guzman.