If ever a book was written in the nick of time, it is “The Lost” (Harper-Collins, 512 pages, $27.95), the hybrid memoir, travelogue, and detective story by Daniel Mendelsohn. Ever since he was a child in the 1960s, Mr. Mendelsohn — a classicist, memoirist, and regular contributor to the New York Review of Books — has been fascinated by the history of his family. For decades, he interviewed his relatives, wrote letters, and perused genealogy databases and Web sites, hoping to trace new branches on his family tree. So far, his quest is one that almost any American could share: We are all the children of immigrants, and the rupture of immigration means that most of us see our distant ancestors only through a haze of rumor and legend. To learn the name of your great-great-grandfather, to know what he did for a living, where his house stood, perhaps, even, what kind of a person he was — for the American descendant of Italians or Koreans or Africans, this is the kind of dream that can become an obsession.

What intrigued Mr. Mendelsohn most of all, however, was the part of his family story that is ineluctably Jewish: the murder of six of his relatives in the Holocaust.His beloved maternal grandfather, Abraham Jäger, was an eager storyteller. But he would almost never discuss what happened to his brother, a butcher in the Galician town of Bolechów named Shmiel, and his wife and four daughters. “Once I heard someone saying, He was one of the first on the list,” Mr. Mendelsohn writes; another time, “I heard someone saying, Four beautiful daughters.” It was a small but unmistakable instance of the mystery — at once shameful, frightening, and provocative — that marks the childhood of so many descendants of survivors. Not until he was 12 years old did Mr. Mendelsohn work up the courage to ask his mother what had happened to Shmiel’s four daughters, only to receive the shocking answer: “They raped them and they killed them all.”

In 2001, after decades of gathering scraps of information, Mr. Mendelsohn began the research expeditions chronicled in “The Lost”: trips to Ukraine, Sweden, Australia, Israel, all the spots where survivors of Bolechow had been hurled by history. And as a glance at the “In Memoriam” page at the end of “The Lost” shows, Mr. Mendelsohn started his travels just in time. Just three or four years after he began writing, most of his interviewees were dead. By rescuing Shmiel Jäger’s memory at the last possible moment, Mr. Mendelsohn ensured that his physical extinction would not be followed by historical extinction — that he would not be completely “lost.”

Yet for all his remarkable success in unearthing his relatives’ fate, the real purpose of Mr. Mendelsohn’s book is to show how partial and fragmentary our knowledge of the past must always remain. “The Lost” is, in fact, mostly a chronicle of frustrations.Thanks to his persistence, the Internet, and a lot of frequent flier miles, Mr. Mendelsohn tracked down the last few people in the world who still remembered Shmiel Jäger and his daughters. But these Holocaust survivors, by definition, had escaped or hidden themselves before the Jägers were killed, and so they could know their fates only from hearsay.

Thus the contradictory stories proliferate: Shmiel, his wife Ester, and his youngest daughter Bronia were killed during the second big Aktion in Bolechów, in September 1942. But no, Shmiel was still alive in 1944, in hiding with another daughter, Frydka. Or else Frydka went with the eldest girl, Lorka, to join the Babij partisans, and was killed when the Nazis wiped them out. Or else Frydka was being hidden by a Polish boy, Ciszko Szymanski, who was in love with her. And she was pregnant with his child — or she was pregnant by someone else — or she wasn’t pregnant at all, and it was a woman, an art teacher, who was hiding her.The narrative suspense of “The Lost” comes from Mendelsohn’s patient sifting of rumors, until something like the truth — never ironclad, but plausible and consistent — finally emerges.

The sheer uncertainty of history, then, is one of the obstacles to Mr. Mendelsohn’s quest.This is never clearer than when he visits Bolechów, the Jägers’ ancestral town — once part of the Austro-Hungarian Empire, part of Poland then, after World War I, and now part of Ukraine — where the street names have all changed, many of the houses are gone, and only a handful of elderly people can even remember a time when Jews lived there. And no wonder — of the 6,000 Jews in Bolechów in 1941, only 50 were still alive in 1945.

Drawing on testimonials and published histories, Mr. Mendelsohn tells the story of how the Holocaust came to this typical Eastern European shtetl. In the Aktion of October 28–29, 1941, some 800 Jews — the wealthiest and most prominent, the community leaders – were rounded up in a community center and tortured: suffocated, beaten, their eyes gouged out, their bodies chopped up and shredded. (The Jews were also forced to form a human pyramid — a horrifying echo of what American soldiers did to Iraqis at Abu Ghraib.) The survivors were then marched to a nearby wood and shot. It was in this Aktion, Mr. Mendelsohn discovers, that Shmiel Jäger’s daughter Ruchele died — at least, she was never seen again after that day. But it is impossible to know exactly when or how she was killed; it can only be imagined, and it is so horrible that it cannot be imagined.

This is the second, and finally the more important, barrier to understanding the Holocaust. Even when Mr. Mendelsohn determines how one or another of the Jägers must have died, there is nothing in his (or the reader’s) experience that can allow him to imagine what their death was really like. “What is the smell of a thousand terrified people being herded to their deaths?” Mr. Mendelsohn writes.” What is the smell of a room in which a thousand terrified people have been kept for a day and a half, deprived of toilets … a room in which perhaps a few dozen people have been shot to death, a woman has gone into labor? I will never know.”

Yet in spite of these insurmountable barriers to knowledge, Mr. Mendelsohn insists on searching and writing. More than just an act of familial piety, this kind of recuperation is one of the distinctive ethical acts of our time, after a century that showed how millions of people can be obliterated. It is the kind of stand against oblivion that Walter Benjamin famously symbolized in the Angel of History: “His face is turned toward the past. Where a chain of events appears before us, he sees one single catastrophe, which keeps piling wreckage at his feet.The angel would like to stay, awaken the dead, and make whole what has been smashed.” Or, as Mr. Mendelsohn puts it toward the end of “The Lost”: “Most everything will be lost, eventually, as surely as most of what made up the lives of the Egyptians and Incas and Hittites has been lost. But for a little while some of that can be rescued, if only, faced with the vastness of all that there is and all that there ever was, somebody makes the decision to look back.”

akirsch@nysun.com