Jedwabne is a small town in northeastern Poland. In September 1939, Hitler invaded the country, and Jedwabne came under German occupation—before being passed to Stalin a few weeks later and incorporated into Soviet Belarus. After Nazi Germany attacked the Soviet Union on June 22, 1941, the Red Army withdrew from Jedwabne, and the Wehrmacht returned. A window of semi-anarchy opened, created by two totalitarian regimes carrying out modern social engineering projects in a not-very-modern space: Jedwabne was rural and impoverished. A third of the townspeople could not read or write.

In early July 1941, as they were still setting up their occupation regime, the Germans gave local Poles a few days to “self-cleanse” the town of Jews. “Self-cleansing” began with rapes, stonings, drownings, killings by farm tools. On the morning of July 10, 1941, Polish townspeople drove the Jews from their homes to the market square, where they forced them to pull the weeds from between the cobblestones. It was a scorching midsummer day. Jewish men were made to destroy the nearby Lenin monument, to carry a large piece of it on two wooden poles into the square, and then to proceed into a barn distant by a few hundred meters. Inside the barn, Poles killed the men and flung pieces of Lenin on top of the corpses. Then some 40 men herded the several hundred remaining Jews—now including women, elderly people and small children—into the same barn. “Some were herding their own schoolmates,” said a woman who was there that day. A short, heavy man named Józef Ekstowicz doused the barn with gasoline; someone lit a match and the barn burst into flames. Babies were tossed inside.

Leon Dziedzic, a local Pole then 14 years old, did not take part in the murder. But a few days afterward, he was among those ordered to clear the bodies, an experience he later related to journalist Anna Bikont: “We tried to figure out who was who,” he told her, “the charred bodies were on top, and toward the bottom they were only slightly singed. When the murderers set the barn on fire, people had all rushed to one air shaft on the eastern side. They were all piled up. It was impossible to say how many there were, because we’d take out an arm and then a head separately, with pitchforks.”

In 1949, under a communist-dominated government, 22 Polish men were put on trial for Nazi collaboration consisting of murdering Jews in Jedwabne; 12 were found guilty. Despite the trial, what happened on July 10, 1941 in Jedwabne would become one of very many things that took place during World War II and the Stalinist years that were not spoken about—this was true not only in Poland.

In 1968, Holocaust survivor Awigdor Kochaw recorded a testimony for the Yad Vashem memorial in Jerusalem. Kochaw, summarized the transcriber, “claimed that the majority of Jews from his village, Wizna, were burned in Jedwabne, and that this was done with German permission, but that they died at the hands of Poles.” The copyist added the note: “I don’t understand this part.”

Nor would nearly anyone else, until the turn of the century. In spring 2000, the historian Jan Tomasz Gross published, in Polish, a small book titled “Neighbors: The Destruction of the Jewish Community in Jedwabne, Poland,” telling the story of the July 10 massacre. For six months after the book’s publication, there was little public reaction. Then the conversation exploded: By the next spring, more than 100 texts about Jedwabne were appearing each month. The Jedwabne debate became the most important debate on the Holocaust to take place in Europe after the fall of communism.

“It is very difficult to describe how great a shock ‘Neighbors’ was,” wrote the former dissident and political prisoner Adam Michnik. Ms. Bikont, a journalist for Gazeta Wyborcza, the newspaper Mr. Michnik founded and edits, wanted to go to Jedwabne and search for witnesses. Mr. Michnik was opposed. For some time, he later wrote, “I thought my friend Jan Tomasz Gross had become the victim of a hoax.” Ms. Bikont insisted. She had experience with uncomfortable topics: In the 1980s, she co-founded and edited the most important underground newspaper in communist Poland. She began traveling to Jedwabne—a 3½-hour drive on two-lane roads from Warsaw—again and again.

“The Crime and the Silence” tells the story of a massacre; it also lays bare the work of an investigative journalist. Ms. Bikont meticulously checked facts and corroborated testimonies. She struggled with the ethics of persuading witnesses to appear under their own names, when she could offer them no protection. She struggled, too, with the ethics of disguising her own identity so as to persuade people to talk to her. Time after time, residents of the town slammed doors in her face. Others asked her why she hated Poland so much—Ms. Bikont, who under communism devoted herself to fighting for an independent Poland, who was among a handful of women who saved Solidarity after Gen. Wojciech Jaruzelski declared martial law in December 1981 and sent the men to prison.

Ms. Bikont did not give up; the quality of her journalism is something very special. An extraordinary interviewer, she developed relationships with the most unlikely cast of characters. These include the brothers Laudański: Kazimierz, Zygmunt and Jerzy, children of a man taken prisoner by the Soviets during the first half of the war. Zygmunt and Jerzy were among the most energetic—and unrepentant—of the murderers in Jedwabne. “We Poles made sure the Jews didn’t run away,” Jerzy testified in 1949. Following the 1948 trial, he served eight years; Zygmunt served six.

When Ms. Bikont found them, the brothers were beekeepers in a town some 60 miles north of Jedwabne, enjoying local fame for the sweetness of their honey. Kazimierz (who may or may not have been in Jedwabne that day) defended his younger brothers. “How can they call my brothers thugs?” he said. “What we did, we did out of patriotism.” There was an explanation: “Our guys acted in self-defense, just like in all the other uprisings, which we’re not ashamed of. But when you make an omelette you’ve got to break some eggs. And since there were some uneducated people there, they might have caused the deaths of a lot of innocent people. But Polish and Jewish Communists were wrong to collaborate with the NKVD. Traitors get their throats cut.”

Ms. Bikont also comes to know Marianna Ramotowska, once Rachela Finkelsztejn, who fled from a similar massacre in the nearby town of Radziłów; and Stanisław Ramotowski, the Pole who, in order to save Rachela Finkelsztejn, made her his wife. Marianna and Stanisław Ramotowski were initially reluctant to talk. It would be impossible to tell the story, Stanisław said; after all, he and Marianna wanted to be able to live in the place where they were living. “I won’t tell you,” he added, “how I arranged with the priest to get married in wartime when I wanted to marry a Jewish girl. A thousand horses couldn’t drag it out of me.” “Don’t say anything, Stasinek, God forbid,” Marianna interjected.

Stanisław and Marianna survived together, but there were costs. In 1942, while in hiding during the Nazi occupation, Marianna gave birth to their child. Perhaps it was the midwife who suffocated the baby at once; Stanisław believed this was for the best: With a baby, they almost certainly would have been discovered by the Germans. Many times he went out drinking with those who carried out the July 10 massacre; in this way he learned all the details of the crime. After the war, Marianna even testified on behalf of local Poles charged with murdering Jews. “We would have helped any of the killers, otherwise we wouldn’t survive,” she finally told Ms. Bikont.

The townspeople defended themselves to Ms. Bikont: the Germans forced them, they insisted, and if they did take some initiative, the Jews deserved it for having welcomed the Red Army. Ms. Bikont often found her interlocutors unable to separate Poles’ sympathy for anti-Semitism and collaboration with the Nazis from Jews’ supposed sympathy for communism and collaboration with the Soviets. In fact—and here Ms. Bikont cites the research of historian Krzysztof Jasiewicz—in the Jedwabne area 126 Poles and 45 Jews openly collaborated with the Soviets between 1939 and 1941, representing 0.34% of the Polish population and 3.2% of the area’s Jewish population, respectively. Many of the murderers had themselves previously collaborated with the Soviet regime, including Zygmunt Laudański. Cases of double and even triple collaboration (with each successive regime) were not unusual. It was also not unusual to find heroes of the Polish resistance who murdered Jews. It was a time when anything was possible.

Under dictator Marshal Józef Piłsudski, the interwar Polish state had been anti-anti-Semitic. The region around Jedwabne, though—Ms. Bikont learned through immersing herself in local newspapers from the 1930s—was a stronghold of the nationalist far right. Village priests, often corrupt, served not only as apologists for, but also as inciters of anti-Semitic violence. In July 1941, Jedwabne was only one among several closely clustered sites of “self-cleansing.” Enthusiasm for looting Jewish property was arguably much greater than enthusiasm for killing Jews; it was a time when a shirt was worth more than the person who wore it—to say nothing of the house where that person lived. “They needed underwear so they took it from the Jews,” Leon Dziedzic explained to Ms. Bikont. His son interrupted him: “Tell her, Dad, like you told me: They went out killing for a s—y Jewish nightshirt.”

The confrontation between Poles and Jews is only one of three dramatic confrontations at the heart of this book. The second is between the generation that took part in the war and the generation that was “graced by a late birth.” How do grown children accept—or not—that their fathers were murderers? Ms. Bikont arrives in late autumn 2000, as if an intruder at an involuntary psychoanalytic session: a whole town experiencing the revenge of the repressed. This intrusion is the third confrontation: between intellectuals from the city and peasants from the countryside. Ms. Bikont comes to the provinces as if to a foreign country.A schoolteacher in Jedwabne tells her: “The only accepted life model here is to put money in the tray on Sunday and then drink all week, beat your wife, and moan about the Jews.” That the anti-Semitism virulent in the middle of the 20th century continued into the 21st was for Ms. Bikont a moral shock.

Not only the murderers, but also the rescuers—who risked their own lives to save Jews—remain terrified to talk: Well over half a century later, they still feared their neighbors. Yet they lived with them and had done so for a long time. This book leaves the reader haunted by the intimacy of the massacre: Women were raped by their own classmates, hundreds of people were burned to death by their own neighbors, and the few survivors who remained spent the rest of their lives among their families’ murderers. Even Stanisław Ramotowski kept doing business with one of the murderers “because there wasn’t another smith who shod horses as well as he did. ‘But I wouldn’t look at him or he at me; he kept his head down.’ ”