For human rights activist Isabel Letelier, the widow of Orlando Letelier, September is the cruelest month.

On Friday morning, she got in the car and drove. It was a gray morning, damp and cool, like the one five years ago that changed her life.

She drove straight to Bethesda, visiting each of the five houses she had lived in with her husband, the former Chilean ambassador. As each came up, she would slow down. "It was like a pilgrimage," she says.

The last house was the one on Ogden Court. "It's not a house that brings the better memories. I was thinking about these guys coming one night and putting a bomb under the car in that driveway . . . "

Five years ago today, Orlando Letelier and Ronni Karpen Moffitt, a co-worker, were killed in what is considered the most notorious act of international terrorism ever to occur in this city.

The sky was overcast . . .

Letelier's car, tailed by the grey sedan, passed the Chilean Embassy on the left and entered Sheridan Circle. The driver of the grey sedan pressed one key on the instrument plugged into the cigarette lighter, then pressed a second key.

Michael Moffitt heard a hissing sound, "like a hot wire being placed in cold water." He saw a flash of light over Ronni's head, then a deafening, crushing sound overwhelmed him. -- from "Assassination on Embassy Row," by John Dinges and Saul Landau.

Michael Moffitt, Ronni Karpen Moffitt's husband, survived the bomb explosion that rocked Letelier's car as he drove it around Sheridan Circle. Letelier was 44 -- a prominent Chilean exile and an outspoken critic of the Pinochet military regime, the ambassador during the three-year presidency of Socialist Salvador Allende, an economist and the director of the international program of the Institute for Policy Studies, a liberal think tank here. Ronni Karpen Moffitt, an IPS fundraiser, was 25.

What has happened in the interim is as much the story of how Letelier's widow, Isabel Letelier, 49, stepped into the waiting limelight as it is the story of how the limelight has flickered a bit on human rights.

"The first time I had to speak, I felt I had to say the things Orlando would have said, the economic analysis," says Isabel Letelier, seated on a couch in her office at IPS, where she is now a fellow. "It was such a terrific pressure on me. Fortunately it didn't last too long. It lasted about four months and I realized I am what I am. I am going to keep doing my human rights work."

Generally, her speeches garner support. "I remind people constantly of something that happened," she says. "Of course, it would be beautiful to live in a city where you are yourself. But then perhaps I couldn't be as effective." She chuckles. "I'm very clear on that."

Her hair is thick and dark -- and full of gray that wasn't there five years ago. Her eyes are luminous and green and she smiles easily. A former teacher, she is now the human rights advocate and the widow and the two are impossible to separate, although she says she doesn't think about it that way.

A year before her husband's death, Isabel Letelier had started the Chile Committee for Human Rights, an informational organization. Since his death, she has been speaking all over the world to all kinds of groups and assemblies. She gives receptions and fund-raisers for Chileans and Haitians and Puerto Ricans and Paraguayans. And when she doesn't show up at a function, hosts grumble, "Where was Isabel?"

"She is a haven for the oppressed," says author Saul Landau, who is also a fellow at IPS. "She cares about all sorts of people no one is interested in. Nobody cares about the Haitians."

One month after her husband's death, she made her first speech -- in Boston. She has a secretary, and file cabinets full of manila folders on where and when she has gone to speak. She rifles through the folders and pulls out a folded poster featuring her photograph -- young and beautiful and cooly defiant. "ISABEL LETELIER IN SEATTLE, Oct. 5." The year was 1977. She studies it noncommittally and shrugs. She can't remember to whom she was speaking.

She is passionate when she speaks and can be moderate when necessary.

She's also realistic. When two anti-Castro Cubans were convicted of the murders in a highly publicized trial more than two years ago and then retried this spring and acquitted, Letelier was quoted the day of the verdict as saying: "I think justice has different ways of showing itself. My husband is not here any more. What can I say? Ronni is not here any more."

She is strong and unflagging and perhaps, always, a little obsessed. Haunted, she says.

Isabel Letelier was picketed by Young Americans for Freedom when she showed up for a speech once in Nebraska. They said she was a communist. "I had experienced violence close to me, but when I saw that picket line and I realized those 30 people didn't like me at all," she says, more bemused than upset, "I thought, 'I'm all alone in this. And this is the only body I have and I intend to keep it.' " She laughs.

Her main goal is to see democracy established in Chile.

It is a country that she is not allowed to return to right now. The last time she was there, she says secret police constantly followed her and her friends. At one point, she got into a taxi with a friend and a man ran up to the door to try to photograph her friend.

Letelier, sensing it would be dangerous for a Chilean to be identified with her, turned to the photographer and shielded her friend. "I told him," she says, eyes wide, voice calm, " 'If you keep pestering me, I'm going to break your balls.' "

In her office are photos of Orlando, posters and plaques. There are snapshots of people standing at memorial services in Sheridan Circle. There is a facsimile of the Letelier-Moffitt Human Rights award, which features images of Orlando Letelier and Ronni Moffitt.

She puffs on a Salem Light ("I'm a nonsmoker who smokes occasionally," she says with a little smile) and glances toward the window.

"This is what I resent," says Letelier. "Ronni Karpen Moffitt has not been properly . . . " She searches in vain for the word. Recognized? She nods. "I would like to see women's groups talking about that. An American woman on her way to work was assassinated by a government that has been supported by this government. I feel it's outrageous. How can we accept it? That's absolutely unacceptable."

Passions soar on Sunday at the fifth annual Letelier-Moffitt Human Rights Award banquet.

One of the recipients of the awards is Jacobo Timerman, the Argentine publisher and critic of the Argentine junta who was jailed and tortured for 30 months without any charges. "What made me decide to come here personally," says Timerman, who lives in Israel, "is to say to those organized in the slander against the institute for Policy Studies : We are not afraid . . . the slander against the institute will not stop my support . . . "

He is cheered enthusiastically.

Timerman is seated between Isabel Letelier and Sister Blaise Luppo, who was accepting the other Letelier-Moffitt award for the Maryknoll Nuns in recognition of their work with the poor, often in dangerous areas. "I think that as Maryknolls, the greatest disappointment are the people here in this country ," says Sister Blaise Luppo as she accepts the award. "We have eyes but we don't see . . . We can dabble in human rights as an intellectual exercise, but few of us ask the question -- why is it that three-fourths of the population is allowed to live a subhuman existence?"

Her voice rises strong through the room in the Georgetown Hotel where about 250 supporters and friends sit squished at tables. At one table, I.F. Stone sits between Rev. William Wipfler -- last year's award recipient -- and Rep. Tom Harkin (D-Iowa). At another table sit Ronni Karpen Moffitt's parents.

When she finishes -- telling the people in the room "you keep us going" -- she gets a standing ovation. Letelier and Timerman each kiss her on the cheek.

"Can you imagine," Letelier says, her voice low, "plotting an assassination in Washington, D.C., on Embassy Row? What was the message of that? That they do whatever they want."

Michael Townley, the American-born Chilean secret police agent who was the government's key witness at both trials, is in prison. The two Cubans indicted for the bombing, were acquitted after the second trial.

"And they knew," she continues. "They saw Michael and Ronni in that car. That's something that haunts me all the time, you know? That there were two Americans there, and they couldn't have cared less . . . Why did they do it there? And why in Washington?" There are no tears, just questions. "I have the feeling that there are many things that are not clear yet -- and we're going to know more about it one day."

Assistant U.S. District Attorney E. Lawrence Barcella is haunted, too. Barcella and Eugene Propper investigated the murders and put together the prosecution of the first trial. He thinks about the Letelier case a lot.

"Just 15 to 20 times a day," says Barcella, who is still working on the case. "I've got a picture of Contreras the fomer head of the Chilean secret police, DINA, who was indicted in the Letelier murder but never extradited on a door in front of my desk, so I can't forget it."

Two other Cubans connected with the case are still fugitives. And Barcella has a question yet to be answered: Who pushed the button?

"I've found it difficult," he says, "to drive down Massachusetts Avenue through Sheridan Circle without thinking about it."

These are tough times for human rights advocates.

"When people like Reagan get elected, you don't hear the sound of checkbooks opening to make contributions to groups affiliated with the left," says Saul Landau, coauthor with John Dinges of "Assassination on Embassy Row."

"Human rights is apparently not in vogue," says Rep. George Miller, a liberal Democrat from California. Miller visited Chile with Orlando Letelier in 1975. About Isabel Letelier, he says: "She continues to hold people who were concerned five years ago to being concerned today. She does it because she picks up the phone. She won't let items pass. She doesn't let you shrug your shoulders. You shrug your shoulders at her and she'll tell you her life story."

It is a life story changed by a painful incident. But so has the landscape changed. Much of the staff at IPS is new. In recent months, the institute has been fighting accusations of subversive activities by a variety of conservative organizations. And the Reagan administration has de-emphasized Carter's strong human rights policy.

Michael Moffitt has changed in five years. "I got very fat and then very thin," he laughs. Moffitt, an economist, is 30 now. He spent the summer on Long Island writing a book about international banking and finance. He and Isabel Letelier are friends, but they were never particularly close, he says.

"I haven't been that closely involved in Latin America and Chile," he says. "I felt it was time to move on. I don't think Isabel has the luxury of that choice. She's been identified with the move to get rid of the Pinochet regime. And he could be there a long time."

Isabel Letelier is willing to wait. "There is a large group of people who will fight until the end," she had been saying in her office earlier in the week. "We will keep working toward justice, toward peace until the very last breath," she says, pausing, "on this earth." She smiles broadly. "I'm 49. I'm just beginning."