When Alfredo Etcheberry goes before the Chilean Supreme court later this month seeking the extradition of three former secret police members, the Orlando Letelier murder case will enter another phase with possible new perchet.

Unless the court changes its traditional procedures or this highly sensitive case, the evidence - not just the charges presented by a federal grand jury in Washington - will become public the instant the request is filed.

For the first time, the public here will have the chance to sift through the facts and weigh the degree, if any, to which ranking members of the government were involved in the 1976 murder of the former Chilean diplomat.

Etcheberry, one of Chile's most skillful lawyers, has been hired by the U.S. Embassy for the extradition proceedings that will determine whether the three accused Chileans are sent to the United States to stand trial for the murder that, more than anything else, has disrupted U.S. Chilean relations.

So far, though there is little evidence that the indictments have stirred great interest among average Chileans, shaken the faith of government supporters or severely damaged the political capital of Pinochet, to whom Chile's old secret police answers personally.

After an initial flurry of attention when the indictment were announced last week interest in the case seems to have died down.

A diplomat observed that there was far more concern, even panic, at the end of June when U.S. Ambassador George W. Landau was recalled to Washington for consultations about the Letelier affair than there was last week when the indictments were finally announced.

Chileans had expected the indictments for months, but Landau's recall was unexpected and seemed as if it might lead to a break in relations, something that would alarm most average Chileans and supporters of Pinochet.

At the moment, however, many Chileans seem to feel that, even if Gen. Manuel Contreras Sepulveda, the former head of the secret police, and two of his subordinates who were charged with the Letelier murder, are found guilty, that alone will not necessarily mean the end of Pinochet or the military government he heads.

"There is no feeling of imminent danger," said another government supporter. "The current government will be in place for at least another three or four years."

There is a nagging fear, however, even among the government's strongest supporters, that Etcheberry's trip to the Supreme Court could change the situation drastically because all the evidence will be made public.

Instead of charges, there will be facts to consider. Presumably, the Chilean public and, most importantly, Chile's other generals and admirals, who will ultimately decide whether Pinochet stands or falls, will assess the strength of the case developed in the United States.

No one here will know for sure if the evidence links Pinochet to the killing until it is released. Then, there is the unlikely possibility would lead to the president's downfall.

It is generally believed that neither average Chileans nor the country's other military leaders could accept the idea that the president ordered what was in fact an act of terrorism.

But most Chileans seem to believe Pinochet when he says he knew nothing about the assassination until it happened.

Even if Pinochet survives the Letelier case, however, his supporters say, it will cost him dearly. Chile's military has a strong sense of honor and it is said that there is already some grumbling within the ranks that the Letelier case has undercut the military's reputation.

The extradition proceedings could take months, and will keep the issue alive. Etcheberry must first demonstrate that the evidence gathered in the United States would be sufficient to indict the three under Chilean law, which requires a higher standards of proof than the U.S. system of justice.

If Etcheberry succeeds, Justice Israel Borquez, who will hear the case alone, still has the option of ruling the Letelier assassination was a political crime - which would mean nonextradition and no further trial here.

If the judge decided that Letelier's death was a common crime, however, he could order a trial here based on the U.S. evidence, instead of ordering the three Chileans to stand trial in the United States.

Interestingly, an absolute defense under Chilean law for a military man accused of a crime is to prove that he was ordered to do it by his superior.

It would be up to Contreras to decide whether to say that a higher authority, in effect, ordered the assassination of Letelier.