MEXICO CITY (Reuters) -Gunmen massacred 20 people on Sunday night in a suspected gangland attack at a clandestine cockfighting venue in western Mexico, authorities said, in one of the worst mass shootings under the current government. The killings took place in Las Tinajas in the state of Michoacan, where the powerful Jalisco New Generation Cartel (CJNG) has been fighting local gangs for control of drug routes. “It was a massacre of one group by another,” President Andres Manuel Lopez Obrador told a regular news conference, expressing his regret at the deaths. He blamed criminal gangs in the a…