Sometimes we think we know something, but we know it only in the most abstract way, which means we may not know it at all.

I can’t say it better than one of Daniel Mendelsohn’s travelling companions does toward the end of this powerful work of investigative empathy: “The Holocaust is so big, the scale of it is so gigantic, so enormous, that it becomes easy to think of it as something mechanical. Anonymous. But everything that happened, happened because someone made a decision. To pull a trigger, to flip a switch, to close a cattle car door, to hide, to betray.”

Others have grappled with this problem: how do you tell the story of the Holocaust in a way that encompasses both its vast geopolitical and its intimately personal dimensions? On the one hand, for instance, there is “The Destruction of the European Jews,” Raul Hilberg’s portrait of the continentwide project of genocide, which includes everything from railway schedulers to Zyklon B gas manufacturers. And there is “The War Against the Jews,” Lucy S. Dawidowicz’s invaluable account of the origins of the extermination in the perpetrators’ ideology. On the other hand there are the memoirs of survivors like Primo Levi and Elie Wiesel, along with numerous less well-known but no less affecting personal accounts. There is also an entire “second generation” literature, both memoirs and novels by children of victims who testify to the enduring questions the Holocaust has left behind, questions about the nature of human nature and the perplexities of theodicy — the relationship of God to the evil visited upon the innocent. There are novels about attempting a new life in the aftermath, like Isaac Bashevis Singer’s icy masterpiece, “Shadows on the Hudson,” and jarring, unconventional works like Art Spiegelman’s “Maus.”

But the problem is more than one of numbers. In his important study “Holocaust Representation,” the philosopher Berel Lang speaks of the “possibility that content may exceed any possible form.” In “The Lost,” Daniel Mendelsohn demonstrates an awareness of this difficulty and has found a form at once hugely ambitious yet intensely engaging in its humility. He attempts to rescue from oblivion a single family’s fate. And in his quest for the details that endow them with life, he draws us more deeply into the experience of the larger catastrophe than we might have thought possible.

Mendelsohn begins his account with a startling childhood experience: at gatherings of his extended family, many of them survivors of the massacred Jewish community of Bolechow, Poland (now Ukraine), “it would occasionally happen that I would walk into a room and certain people would begin to cry.” They would cry because of his startling resemblance to a distant relative among the dead, his great-uncle Shmiel Jäger, a meat shipper in Bolechow who was murdered by the Nazis along with his wife, Ester, and their four daughters.

Mendelsohn’s initial quest to find out what actually happened to Shmiel and his family — the details of the fate they shared with millions — begins with normal, if death-inflected, genealogical curiosity. But when he discovers a series of increasingly desperate letters from Shmiel to his family in America, written in 1939 as events close in on Bolechow’s Jews, he is drawn into an obsessive multi-continent search that recreates the life of the Lost as well as their annihilation. The result is a new way of telling a story we thought we knew.

It seems that all the while he was writing lucid essays reflecting his classical scholarship for The New York Review of Books and other publications, Mendelsohn was doggedly tracking down relatives and survivors of Bolechow in search of memories of Shmiel, his wife and his daughters. The fact that his search begins with a physical resemblance to one of the Lost suggests that an important aspect of the obsession is a search for his own identity, his face in the face of the Loss. The tears he provoked as a child grow in this saga into a meditation upon “the tears of things”; the sorrow of one family becomes the sorrow of us all.

In the beginning the reader may feel a little at sea in the welter of details and the web of relationships — whose great-aunt was the sister of which brother-in-law? It is a tribute to Mendelsohn’s narrative skills that one soon finds the close focus on family details absorbing, novelistic. Before long one begins to grasp Mendelsohn’s method, which draws on both the classical and the Biblical modes of storytelling. In fact, his interspersed meditations on conflicting models of storytelling are one of the most thought-provoking and original features of the book.

How has the literature of the past coped with inconsolable tragedy from the flood to the fall of Troy? Homer, he reminds us, “will often interrupt the forward motion of the ‘Iliad,’ his great poem of war, spiraling backward in time and sometimes space in order to give psychological richness and emotional texture to the proceedings, or to suggest, as he sometimes does, that not knowing certain stories, being ignorant of intricate histories that, unbeknownst to us, frame the present, can be a grave mistake.” And so Mendelsohn himself spirals back by means of letters, diaries and family photo albums, which cumulatively recreate the world of prewar Bolechow, a place caught between medieval spirituality and modernity, a lost world once shared by Jews, Poles and Ukrainians in deceptive comity.

Mendelsohn does something rare in the Holocaust literature I’ve read, which is to see the lives of the future victims not as completely “backshadowed” (in the scholar Michael André Bernstein’s phrase) by the tragedy to come, but illuminated by their innocent ordinariness. I was particularly struck by a photo of Uncle Shmiel standing next to his prized meat-shipping truck. (Mendelsohn makes excellent use of old photos and also new ones of the aging Bolechowers taken by his brother Matt.)

At times one wonders if Mendelsohn is pressing his relentless search for details about Shmiel and his family beyond the point of relevance. For example, there is the moment late in the book when he devotes several pages to conflicting testimony on whether one of the murdered teenage daughters was “easy,” in the pejorative sense of the word. But just as one is on the brink of asking why this matters, Mendelsohn turns the inquiry into a moving tribute to the daughter. He asks whether, beneath the various versions, there is “a story of a willful and beautiful teenager, rather tall and perhaps a little bit spoiled, a girl whose flighty and egotistical personality, subjected to the tremendous and crushing pressures, to the unimaginable forces of hardship and suffering and grief under the Occupation, had metamorphosed into something heroic and brilliant.” So the inquiry is less a gossipy digression than a deeply touching account of transformation in the face of tragedy.

Overlaying the Odysseyan wandering and spiraling of this narrative is a different kind of structure, specifically the tripartite structure of the narrative of Genesis that begins with creation, builds to catastrophic annihilation (the flood), and then moves on to the recreation or reconstruction of human society. Indeed, one of the distinctions of this book is the way Mendelsohn wrestles with the tradition of commentary on Genesis and relates it to the attempt to represent the Holocaust.

Consider, for example, his commentary on the commentaries on the story of Lot’s wife, who was warned not to look back on the fiery destruction of Sodom and Gomorrah. Of course she does turn around and is turned into a pillar of salt. Mendelsohn believes sages like Rashi and other commentators miss the emotional appeal and peril of the backward glance. But Mendelsohn sees the episode as a warning that “regret for what we have lost, for the pasts we have to abandon, often poisons any attempt to make a new life.” For those compelled to look “back at what has been, rather than forward into the future,” he writes, “the great danger is tears, the unstoppable weeping that the Greeks ... knew was not only a pain but a narcotic pleasure, too: a mournful contemplation so flawless so crystalline, that it can, in the end, immobilize you.”

It’s a sentiment that can seem like a challenge to his entire enterprise. But Mendelsohn also seems to suggest that we can’t look forward until we look back, until we know how we came to be who we are — until we know what we have lost. He tries to look back — to see the horror of annihilation — through the eyes of the single family he has brought back to life. He speaks of “what it would have been like, as a 16-year-old, perhaps oversheltered girl of a certain era, to witness other people being killed, tortured, raped, shot. To watch, for instance ... as the rabbi you have known since you were a young child has his eyes cut out, has a cross cut into his chest, and is then forced to dance naked with another terrified young woman.”

And if one thinks one has lost one’s capacity for horror at the depths of human nature, consider this, from an eyewitness deposition he finds about the second roundup of Bolechow Jews: “A terrible episode happened with Mrs. Grynberg. The Ukrainians and the Germans who had broken into her house found her giving birth. ... When the birth pangs started she was dragged onto a dumpster in the yard of the town hall with a crowd ... who cracked jokes and jeered and watched the pain of childbirth. ... The child was immediately torn from her arms along with its umbilical cord and thrown — It was trampled by the crowd and she was stood on her feet as blood poured out of her with her bleeding bits hanging.”

It’s an episode that raises questions about the limits of representation. After giving us an excerpt from another eyewitness account — of Jews packed in cattle cars on the way to the death camps — Mendelsohn writes: “Whatever we see in museums, the artifacts and the evidence, can give us only the dimmest comprehension of what the event itself was like. ... We must be careful when we try to envision ‘what it was like.’ It is possible today, for instance, to walk inside a vintage cattle car in a museum, but ... simply being in that enclosed, boxlike space ... is not the same as being in that space after you’ve had to smother your toddler to death and to drink your own urine in desperation, experiences that visitors to such exhibits are unlikely to have recently undergone.” Well intentioned as such exhibits are, the power of eyewitness testimony suggests that sometimes words are worth a thousand pictures.

The final section of “The Lost” is the most surprising. By this time Mendelsohn has traveled back and forth, often twice, to key places in his narrative, earnestly badgering Bolechow survivors in locales ranging from Australia to Sweden. And — fulfilling the third part of the Biblical structure, the re-creation of the world — he has managed to create, through his linkages of the survivors, in person and by e-mail, a virtual Bolechow, a global ghost town of aging and dying eyewitnesses. But in the end, his investigation narrows dramatically to seek the truth of a single act, a single decision involving hiding and betrayal. “The saviors,” Mendelsohn says, “were, in their way, as inexplicable and mysterious to me as the betrayers.” Why did some help and some betray? The disclosure of the solution to this mystery takes on a powerfully suspenseful momentum as all of the evidence and eyewitnesses of the virtual and real Bolechow are focused upon bringing the truth to the surface.

It is one story, and yes, “the Holocaust is so big.” But Daniel Mendelsohn has invented a unique way of making it, once again, all too real.

Ron Rosenbaum is the author of “Explaining Hitler” and “The Shakespeare Wars.” He writes a column for The New York Obserber.

Copyright 2006 The New York Times Company