When the letters to the editor began to appear late last month, no one was quite sure what to make of them.

Chileans of all political persuasions are easily convinced that the U.S. Central Intelligence Agency is responsible for almost everything that happens in their country, so it did not seem unusual that some Chileans would accuse the CIA of being involved in a plot to blame the assassination of former Chilean diplomat Orlando Letelier on Chile's old secret police force, DINA.

One of the first letters along these lines appeared in the Aug. 30 issue of Ercilla magazine. It was signed by Juan Carlos Olmeda, who accused the CIA, the FBI, and the Carter administration of "fabricating the evidence" used last month to charge three former DINA officers with ordering or helping to carry out Letelier's assassination in Washington two years ago.

The "brilliant idea," according to Olmeda, was to further discredit the five-year-old rightist military government of Chilean President Augusto Pinochet, which has been widely accused of massive human rights violations, by falsely blaming it for the 1976 Chilean ambassador to the United States under Pinochet's leftist predecessor, Salvador Allende, and one of Pinochet's most articulate and effective opponents.

Olmeda does not say who he thinks did kill Letelier, if agents paid by DINA did not, but he does suggest that "the true assassins - which might or might not have included [Fidel Castro's] police - are happy that . . . the CIA has made things worse in the world, particularly for Chile."

Had the letter been unique or one of only a few it might be dismissed as the view of one individual Chilean who does not want to believe that his government could have been responsible for a political murder, especially in the capital of the United States.

But letters such as Olmeda's have been appearing regularly for the past several weeks in a wide variety of publications here, leading some observers to conclude that Chile's military government has mounted a subtle but important campaign to discredit in advance the evidence against the three DINA agents now charged in the Letelier case.

The evidence, gathered by U.S. investigators and delivered Wednesday to the Foreign Ministry here, will become public next week as part of extradition hearings for the three Chileans requested by the United States.

Although the U.S. Embassy in Santiago regards the extradition request as a purely judicial matter, which technically it is, almost everyone here recognizes the important political repercussions that would result from a finding that Chile's military government was responsible for killing Letelier and Ronni Moffitt, an American who was riding in Letelier's car when it was blown up two years ago yesterday.

The formal extradition decision will be made by the president of Chile's Supreme Court, Israel Borquez, but the more important political decision about the culpability of the government will be made by Chile's 10.5 million people.

That is why public opinion is crucial and why supporters of the government seem so anxious to sow doubts about the evidence soon to be presented against the three DINA officers - Gen. Juan Manual Contreras Sepulveda, who headed the secret police; Pedro Espinoza Bravo, who served as its operations director, and Armando Fernandez Larios, an agent who allegedly helped arrange Letelier's death.

If the public here believes the evidence, the military government will be viewed as guilty of terrorist acts of the kind it has used to justify the harsh, authoritarian internal security measures it has imposed on Chile for the past five years. Even if the military remains in power, it will have lost its moral authority.

If, on the other hand, the evidence is not persuasive, the government could emerge stronger than ever. It would not be hard for it to suggest then that the charges in the Letelier slaying were engineered by the Carter administration in a vain effort to replace the military government here with a democratically elected one. Supporters of the government believe - and the government's opponents hope - that this is indeed the U.S. administration's policy.

In an interview last week, Gen. Cesar Mendoza Duran, head of Chile's national police force and a member of the military junta, said he believes that Michael V. Townley, the DINA agent who has provided much of the evidence in the Letelier case, "is an agent of the CIA, the KGB and at the same time an agent of Cuba."

Mendoza describes the Letelier case as a souffle. Said he: "It rises, it goes over the sides and then it falls."

Many other Chileans are not so sure.