The murder in May 1993 was billed by the authorities as either an accident - 'he was caught in crossfire between rival drug gangs' - or mistaken identity - 'he was mistaken for a drug baron'.
Revelations over the past few days, however, have heightened suspicions among Catholic churchmen and the public that Cardinal Posadas was deliberately targeted. The big question is why?
It is a question causing shock waves in Mexico's Catholic church, has led to calls for the expulsion of the Pope's diplomatic representative and become an issue in the tense run-up to what are expected to be Mexico's closest- ever presidential elections on 21 August. Many Mexicans believe hardline elements of the long-ruling Institutional Revolutionary Party (PRI), or even in the Church, may have had a hand in the killing.
Speculation is rife that the murder was the result of links between drug traffickers and Mexican government officials, or even between the druglords and elements of the Church. Prominent Protestant churchmen have publicly accused the Church of collusion with drug cartels and called for the expulsion of Girolamo Prigione, the Papal Nuncio, long a controversial figure. Opposition politicians have joined in the call.
Cardinal Posadas's successor in Guadalajara, Archbishop Juan Sandoval, said this week he knew of witnesses who would back his belief that his predecessor was deliberately murdered. The controversy has heightened a split within the Church between conservatives, symbolised by the Nuncio, and liberation theologists, including radical priests in poverty-stricken areas like the state of Chiapas.
After the cardinal's murder, the authorities blamed two of Mexico's most wanted cocaine and marijuana traffickers, the brothers Ramon and Benjamin Arellano Felix, of the Tijuana cartel. They had been trying to kill a rival drug baron, Joaquin Guzman, went the official version.
Last month, however, the brothers revealed that they had separately visited Mr Prigione in his Mexico City nunciature - the equivalent of a Vatican embassy - in December last year and January this year to deny killing the cardinal.
The Nuncio they said, had passed their message on immediately to President Salinas de Gortari and allowed them out the back door of the nunciature to avoid security guards. The Nuncio confirmed the meetings but said he had kept them secret because of professional confidentiality. News of the Nuncio's secret meetings with two of the country's most- wanted men stunned Mexico.
The combined facts now worrying Mexicans are these:
Cardinal Posadas was at Guadalajara airport to meet the Nuncio when he was shot;
Despite the ensuing mayhem, the gunmen who hit the Cardinal with 14 AK-47 rounds escaped by boarding a scheduled flight to Tijuana, which appeared to have been held up to await them;
It was in Tijuana that Luis Donaldo Colosio, the PRI's presidential candidate, was shot dead less than a year later. When a government- appointed prosecutor concluded he had been killed by a lone gunman, so strong was public disbelief that President Salinas was forced to appoint a new investigator. The gunman, Mario Aburto, was known to have had ties with local PRI officials in the border town and was able to get close enough to Mr Colosio to put his pistol against his temple;
The Arellano Felix brothers' powerful Tijuana cartel is widely suspected of receiving protection from at least some sectors of the PRI and the local government in Tijuana;
Cardinal Posadas had been a strong critic of drug traffickers and particularly of what he considered their links with PRI, government and police officials. Since his death, Mexican drug trafficking has spiralled while police successes have plunged. The amount of cocaine seized so far this year is about a quarter that seized in the first half of last year;
Throughout his 16 years in Mexico, the Nuncio has been involved in a feud with Mexico's most senior churchman, the Archbishop of Mexico City, Ernesto Corripio Ahumada. Archbishop Corripio accuses the Nuncio of interfering in Mexican church affairs and has been trying for years to have him removed;
The Papal Nuncio had been involved in a serious rift last year with the Bishop of the Chiapas town of San Cristobal de las Casas, Samuel Ruiz. He suggested Bishop Ruiz should be excommunicated for pressing his liberation theology on Indian peasants in Chiapas. Bishop Ruiz's strong sermons appeared to fuel the Chiapas uprising and conservative politicians and churchmen have accused him of direct involvement, even of supplying arms to the guerrillas, something he has denied.