"Companero," Isabel Letelier began. Dressed in white, she read her poem in a church, softly lit by Sunday afternoon sunlight filtered through stained glass windows. "We've made it for a year . . ."

Several hundreds friends and well wishers - Chileans, Americans and others - listened in silence. They had come to mourn the death of her husband in silence. They had come to mourn the death of her husband. Orlando - one year ago today - and to join her in denouncing the Chilean military junta that had previously imprisoned him for a year. They had gathered also to grieve for Ronni Moffitt, Letelier's colleague who died along with him.

Letelier, 44, had been ambassador to the United States and a cabinet minister in the Marxist government of the late Chilean President Salvador Allende. Moffitt, 25, had been a staff member of the Institute for Policy Studies here, where Letelier worked before his death. Both were killed where a bomb exploded beneath their car as they drove through Sheridan Circle NW near the current Chilean ambassador's residence on Embassy Row last year. No one has been charged with the slayings.

In the past year since then, a federal investigation has been under way, probing a shadowy network of Latin American conflict, including alleged terrorist activities of anti-Castro Cubans. Protest has continued against the Chilean regime of Gen. Autusto Pinochet. The Chilean government has largely rejected such criticism.

Isabel Letelier and Michael Moffitt, Ronni's husband, have, meanwhile, sought to carry on - speaking widely and writing harshly in opposition to the Pinochet regime. Mrs. Letelier, a sculptress before her husband's death, has had no time for sculpture in the past year, she said a few days ago, largely because of her efforts as president of the Chile Committe for Human Rights.

"In a sense, your work becomes your life," Michael Moffitt noted in an interview last week. "There's nothing in my private life that I would care to do," he added. "I would like Pinochet to know that I spend my life fighting the junta . . . He killed my wife, too."

The memorial service at Holy Trinity Church, on 35th Street NW, last Sunday was among several tributes to Letelier and Moffitt taking place this month. In addition, a Letelier-Moffitt Memorial Fund for Human Rights has been announced. A traveling van is scheduled to be sent around the country to advocate human rights causes in Chile, Argentina and Uruguay.

A national coalition is also being formed to press for completion of the federal investigation of Letelier's and Moffitt's deaths and for prosecution of their killers. Mrs. Letelier and Michael Moffitt said in interviews.

At the time of his death, Letelier, who had formerly been an economist for the Inter-American Development Bank, was at work, along with Michael Moffitt, on an economic study of relations between industrialised and less developed nations.A pamphlet embodying the first part of their research was completed by Moffitt and published during the summer. It found that the economically advanced countries had responded only with "paternalistic pledges" to the poorer nations' demands.

At his death, Letelier was also the most prominent Chilean opponent her of the Pinochet regime. In this role, some of his political allies say, Letelier has been found to be irreplaceable.

"The magic of Orlando was that he could associate with all people opposed to the Pinochet dictatorship," Joe Eldridge, director of the Washington Office on Latin America, a human rights group, remarked the other day. "Nobody's filled his shoes. There's a big vacuum in Washington."

Amid the political crosscurrents, the Justice Department and FBI - with some aid, apparently, from the CIA - have slowly been assembling evidence in their probe of the bombing. According to informed sources, investigators now believe that the murders were carried out by anti-Castro Cubans at the direction of Chile's former secret police organization, known as DINA.

This disclosure - clearly embarrassing to the Chilean governemnt - occurred earlier this month while Pinochet was in Washington for ceremonies marking the signing of the Panama Canal treaties. He was questioned about Letelier's murder during a news conference.

"I can swear that no one in the Chilean government planned anything like that," Pinochet replied. "Letelier was detained in Chile, and I was the one who gave him his freedom and an authorization to leave the country freely."

The federal investigation of the bombing, according to informed sources, has relied heavily on FBI informants in Cuban communities in the United States. The probe has been impeded, however, by the refusal of some prospective witnesses to testify before a grand jury here.

It remains uncertain, informed sources have said, whether the investigation will eventually lead to indictments or only to an internal government report. Such a report, sent to the White House and the State Department, might, these sources say, describe how the bombings took place and why the killers cannot successfully be prosecuted.

Ranging from Miami, Fla., and Union City, N.J., ot Caracas, Venezuela, the federal investigatio has already touched on some of the most prominent men in anti-Castro Cuban circles.

In Miami, investigators have focused on some members of Brigade 2506, an association of veterans of the abortive Bay of Pigs invasion of 1961.