It was, as Cynthia Lucero explained to family and friends shortly before she started the Boston Marathon, ''an extraordinary time,'' her ''week of triumph,'' one of the few moments in life when ''everything comes together.''

Her family had traveled from Ecuador to help her celebrate the completion of her doctoral dissertation last week. The topic: how marathons help runners grieve for lost ones.

On Monday, her parents, who speak little English, were positioned at the finish line to celebrate their daughter's fund-raising run for cancer patients.

But Lucero, 28, never finished the race.

Shortly before 5 p.m. on her way into Cleveland Circle, Lucero stopped, grew wobbly-kneed, and fainted. By the time she arrived at Brigham and Women's Hospital, she was close to comatose.

Last night, life support systems were removed, and a woman who had competed for the benefit of others became the second runner in the 106-year history of the race to die.

''At this point it is impossible to say why she died,'' Dr. Marvin Adner, the marathon medical director, said last night. ''There could be a dozen reasons. A seizure, heat stroke, spontaneous bleeding; she could have hit her head falling to the street.''

Eight years ago, a Swedish runner, Humphrey Siesage, died of a heart attack at the finish line; he was the marathon's first fatality, Adner said.

Hospital officials last night declined to elaborate on the cause of Lucero's death. It was not known if the medical examiner's office will weigh in on the case today.

Friends and family attended a private memorial service at Sacred Heart Church in Roslindale last night.

Family members declined to be interviewed, but friends said that after Lucero's organs are removed for donation, her body will be cremated, then returned to Ecuador for burial on her grandfather's land.

And undoubtedly there will be some friends who do not know of the tragedy and are expecting an e-mail from the runner that now will never come. ''This is my last e-mail before the marathon,'' Lucero wrote supporters of her run who pledged some of $500,000 expected to be raised for leukemia and lymphoma research. ''I will send you a final report after the marathon. It makes me a little nervous to wonder what I'm going to say.''

Lucero had three passions, according to friends: running, clinical psychology, and volunteerism. ''Cynthia was just one of those good people who was always doing the right thing,'' said Carole Slipowitz, a friend for many of the half-dozen years Lucero lived in the Boston area.

She was accepted into the doctorate program at the Massachusetts School of Professional Psychology, a 160-student school in West Roxbury, four years ago. A Big Sister, she also volunteered for the Shelter for Battered Women, the Cambridge Health Alliance, and the South End Community Health Center, and counseled families of victims at Logan International Airport after the Sept. 11 terrorist attacks.

''The contribution this woman would have made to society is staggering,'' said Ethel Magidson, dean of faculty at Lucero's school.

On Monday, Lucero was one of 150 runners raising money for the Massachusetts chapter of the Leukemia and Lymphoma Society. It is one of the 15 official marathon charities whose contributions are expected to top $6 million this year, according to marathon organizers.

''She was a hard trainer, and very much ready for the race,'' said Rick Muhr, who served as the coach for the leukemia team. Two years ago, Muhr accompanied Lucero on her first, and only, prior marathon in San Diego. Lucero completed the 26.2-mile race in about 41/2 hours, he said.

''At first, running a marathon sounds too demanding, if not absolutely impossible to accomplish,'' Lucero wrote in the introduction to her 100-page dissertation. ''But I believe this is not only a realistic goal to accomplish but also an excellent coping mechanism that helps runners and many cancer patients and their families.''

Lucero explored the effects of marathon training on people who have family or friends with cancer. Her thesis is that runners can use the training to help cope with the pain of a cancer diagnosis or the death of a loved one.

Late Monday afternoon, at the bottom of Heartbreak Hill in Newton, the small, compact woman, wearing bib number 15,611 was drinking lots of Gatorade and, as one friend put it, ''looking fine.'' But by the time Lucero had passed Boston College with Cleveland Circle in sight, her demeanor had noticeably changed. Slipowitz saw Lucero was walking. She jumped in beside her and said, ''You don't look well.'' ''I feel dehydrated,'' Lucero answered. And then the marathoner stopped, ''rubber-legged,'' Slipowitz said, and fell to the pavement.

The ambulance was there in moments, taking away Cynthia Lucero, defeated by a race in which she saw such potential for victory.