MEXICO CITY, May 24 (AFP) - Mexican authorities said Monday they were investigating whether the mysterious killing six years ago of Cardinal Juan Jesus Posadas was the outcome of a plot, rather than a mistake as previously believed. Posadas was shot to death May 14, 1993, apparently as members of two rival drug cartels exchanged fire in the parking lot of Guadalajara airport, 450 kilometers (280 miles) west of Mexico City. Deputy state prosecutor Jose Luis Ramos Monday presented a report of investigations, which mentions the possibility that "a non-identified group" had organized the exchange of fire and taken advantage of the moment to kill the cardinal. Authorities earlier suggested the case may have been one of mistaken identity or that Posadas may have been caught in the cross-fire. Earlier this month, Guadalajara auxiliary bishop Jose Gonzalez said he knew who had slain the cardinal and would disclose the killer's name if authorities failed to tell the truth about the case. Gonzalez said solving Posadas' case would be a first step toward explaining other killings that shook the country in 1993 and 1994 -- when presidential candidate Luis Donaldo Colosio and ruling Institutional Revolutionary Party (PRI) leader Francisco Ruiz Massieu were assassinated.  