On April 2, 1979, a day with a northerly breeze, a man named Vasily Ivanov stepped outside in the Soviet city then known as Sverdlovsk to walk his dog. Days later, he fell ill, along with dozens of other people in his town in the Ural Mountains. Their symptoms at first suggested pneumonia or the flu, but soon a darker reality emerged.

“All the soft brain tissues were permeated with blood,” Faina Abramova, a pathologist who examined Ivanov’s body, later told the Los Angeles Times. “In more than 30 years of work, I had never seen anything of the kind happen to flu patients. What I was looking at was utterly different.”

Ivanov was one of at least 64 people killed in an anthrax outbreak that the Soviet government for years blamed on contaminated meat. U.S. intelligence officials doubted the account, but only in the early 1990s were their suspicions confirmed through sleuthing led by a husband-and-wife team of American researchers, Matthew Meselson and Jeanne Guillemin.

Dr. Guillemin, who has died at 76, spearheaded the fieldwork revealing that the anthrax outbreak in Sverdlovsk — now Yekaterinburg — was not the result of infected beef, but rather an accident at a laboratory known as Compound 19, where the Soviet military conducted research on biological weapons in violation of an international treaty that had taken effect in 1975.

Shortly after the fall of the Soviet Union in 1991, she made the first of three trips to Russia, going door to door with interpreters to interview the families of the victims. With information gathered in those interviews, she compiled a map showing the victims’ likely whereabouts when a leak occurred at the lab, sending anthrax spores into the northerly wind — one of the worst accidents in the history of biological weapons.

The locations fell into what Meselson described as a “narrow plume” beginning at Compound 19 and leading to the city limits. Beyond the city limits, sheep and cows had contracted anthrax.

“There was one day, and only one day, when the wind was blowing in that direction all day long,” Meselson said in an interview. “Meat does not travel in a direct line with the wind.”

Dr. Guillemin, who chronicled their work in the 1999 book “Anthrax: The Investigation of a Deadly Outbreak,” described her research as “both an emotional and an intellectual experience.”

“The families did not know why their loved ones had died,” she told the Times in 1994. “I can’t think of a single interview in which they did not cry. I cannot think of an interview in which we did not cry.”

Dr. Guillemin and Meselson — a world-renowned microbiologist known both for his discoveries related to DNA and his successful efforts to enact an international ban on biological arms — joined their colleagues in publishing their results in the journal Science in 1994.

“Meselson, Guillemin and their team had not gone inside Compound 19 nor identified the precise reason for the outbreak, but they peeled away the secrecy that the U.S. government could not penetrate in years of official diplomatic protests to the Soviet and Russian leaders,” Washington Post journalist David E. Hoffman wrote in “The Dead Hand,” a Pulitzer Prize-winning history of the Cold War arms race. “They found solid evidence that anthrax spores had come from the military facility at Compound 19.”

In subsequent years, Dr. Guillemin became a sought-after analyst of biological warfare, blending anthropology, sociology and security studies into a distinctive expertise in a field where men had long outnumbered their female colleagues. She died Nov. 15 at her home in Cambridge, Mass, according to her husband, who said the cause was cancer.

Gregory Koblentz, director of the biodefense graduate programs at George Mason University, lauded Dr. Guillemin for what he described as “pioneering work” spanning “all the major episodes in the history of biological weapons.” He credited her with dismantling some of the “myth and misperception” that has surrounded the topic and with making it accessible to general readers as well as specialists.

Trained in psychology, anthropology and sociology, Dr. Guillemin turned her attention to biological weapons in the 1980s after marrying Meselson, a Harvard University professor who received a 2004 Lasker Award, considered the American equivalent of the Nobel Prize, for his work on genetics and his effort to stop the spread of biological and chemical weapons. Dr. Guillemin taught from 1972 until 2005 at Boston College and had been associated since 1999 with the MIT Security Studies Program.

The investigation in the former Soviet Union was one of several high-profile probes that the couple undertook together. In 1987, they and a British colleague, Julian Robinson, published a report in Foreign Policy magazine challenging accusations by the Reagan administration earlier that decade that Vietnam had used a Soviet biological agent known as “yellow rain” against rebels in Laos and Cambodia. The substance was not a biological weapon, they proposed, but rather bee feces.

After the terrorist attacks of Sept. 11, 2001, when letters containing anthrax were mailed to prominent media figures and lawmakers in the United States, Dr. Guillemin appeared frequently as a commentator in the news, often seeking to quell the hysteria that arose surrounding the episode.

“I’m leery of all the fear­mongering,” she told Salon.com at the time. “America is very healthy. Anthrax goes for older people. Smallpox goes for the weak. We could afford to focus more on other risks.”

Her last book, “Hidden Atrocities: Japanese Germ Warfare and American Obstruction of Justice at the Tokyo Trial” (2017), examined the failure of the post-World War II military tribunal to investigate the use of biological weapons by the Japanese against the Chinese.

Amid Cold War tensions with the Soviet Union, “U.S. military intelligence had actively invaded the Tokyo trial proceedings and suppressed an entire category of war crimes perpetrated against Chinese civilians,” she wrote. “The rule of law could hardly have been more abused in the name of national security.”

Jean Elizabeth Garrigan — she later changed the spelling of her first name to Jeanne — was born in Brooklyn on March 6, 1943. Her father, a businessman, and her mother, a homemaker, later moved the family to Rutherford, N.J., where Dr. Guillemin was educated by Dominican nuns.

She received a bachelor’s degree in social psychology from Harvard in 1968 and a PhD in sociology and anthropology from Brandeis University in 1973. Her doctoral dissertation, about the efforts of a Native American community to adapt to life in cities while also maintaining their cultural traditions, was published as the volume “Urban Renegades” (1975).

Another early book, co-written with Lynda Lytle Holmstrom, was “Mixed Blessings: Intensive Care for Newborns” (1986), about the dilemmas that doctors, nurses and parents face in caring for infants who require treatment in NICUs. Dr. Guillemin’s husband attributed to her Catholic upbringing her interest in the intersection between biology and morality, which would fuel nearly all her work.

She was divorced from her first husband, Robert Guillemin, and pursued her early career while also raising their twin boys as a single mother. In 1986, she married Meselson. In addition to her husband, of Cambridge, survivors include her sons, Robert Guillemin of Brookline, Mass., and John Guillemin of Warsaw; a stepdaughter, Zoë Forbes of Newton, Mass.; two sisters; a brother; and five grandchildren. Another stepdaughter, Amy Meselson, died in 2018.

In the latter years of her career, Dr. Guillemin organized seminars for female graduate students at MIT in an effort to help women advance in security studies. She established an endowment at the MIT Center for International Studies to support female PhD candidates and “energize their sense of inquiry and search for knowledge.”

In her book on the disaster at Sverdlovsk, Dr. Guillemin recalled poring over photographs of the victims. One of them, she wrote, “in her high-necked black dress, with her hair swept up,” looked “like a duchess in a Turgenev novel.” Another, she remarked, had “the cynical expression of an old soldier who has been through many battles, but his eyes are kind.”

They were “just ordinary people,” Dr. Guillemin once told a reporter for the New York Daily News, “all of them in the wrong place at the wrong time.”