By MILT FREUDENHEIM and KATHERINE ROBERTS

United States courts long ago established an official Chilean role in the 1976 murder in Washington of Orlando Letelier, an exiled opponent of President Augusto Pinochet. But the case has never been resolved because Chile has refused to extradite the suspects.

Last week the trial resumed in Washington and a remorseful former agent of the Chilean secret police took the stand.

Armando Fernandez Larios confessed that he had played a role in the plot, traveling to Washington on a false passport in 1976 with orders to find Mr. Letelier, who was a former Ambassador to the United States, and report on where he lived and worked.

Mr. Fernandez, who said he was consumed by guilt, pleaded guilty to acting as an accessory to the death of Mr. Letelier, who was killed with a research associate in a car bombing on Washington's Embassy Row. Michael V. Townley, an American who was a Chilean agent, confessed that he planted the bomb and served 62 months in prison.

In exchange for his confession, longstanding murder charges against Mr. Fernandez were dropped. Reagan Administration officials said that the 37-year-old Chilean had long wanted to come forward but that Chilean authorities told him to cover up his role and that General Pinochet forbade him to return to the United States.

Mr. Fernandez's appearance follows months of arrangements with American officials. He still faces a prison term of up to 10 years on the charge of acting as an accessory after the fact.

Chile has repeatedly refused to extradite Gen. Juan Manuel Contreras Sepulveda, the former secret police head.