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Moisés Escamilla May, 45, had been serving a 37-year sentence at the Puente Grande maximum-security prison in Jalisco state, BBC reported on Monday.
He was viewed as a highly dangerous inmate, having allegedly been responsible for the decapitation of 12 people in the Mexican state of Yucatán.
Widely known by his nickname, "Gordo May," he led a group within the Los Zetas drug cartel called "Old School Zetas."
At the time of his arrest in 2008, May's group was considered the most feared criminal organization in the Cancún area.
May died in prison on Friday. His death was not officially announced until Sunday.
As of Monday, there have been more than 35,000 cases of COVID-19 in Mexico, with some 3,465 deaths, according to the latest figures by Johns Hopkins University. Health experts say the actual numbers are likely much higher.