One day in 2009, Wendy Lower, historian of Nazism and the Shoah, was hard at work in the archives of the U.S. Holocaust Museum when a librarian handed her a photograph dated Oct. 13, 1941, depicting the deaths of Jews in Miropol, Ukraine, a shtetl near Kiev.The photo Lower saw that day is shocking. A woman in a polka dot dress is leaning over from the waist, her head wreathed in smoke from the rifle blasts that are killing her. She is holding the hand of a small boy, who leans backward, his face turned slightly away from her, as if he can’t bear to look into her dying face. The woman grasps his hand tightly. She cannot calm his terror, but she won’t ever let go of him. She is pulling the boy forward with her into the mass grave, along with another child hidden in her lap.We may feel guilty about looking at an image like this, worried that our gaze strips the victims of their humanity. But we are not guilty: It is our duty to look. The photograph that spurred Lower’s search is not mere “atrocity porn,” satisfying an appetite for horrors. Instead, such a photograph wants us to be troubled by the terror it depicts, and to inquire further. The crumpled bodies bend away from the viewer, their pain hidden along with their faces.The shooters in the photo are a German and a Ukrainian—we can see this from their uniforms—and the Ukrainian’s rifle is just a few inches from the woman’s head. Another Ukrainian in the foreground has a rifle in his hands, and there is another German in the back. There is someone else too. “A civilian onlooker in a wool cap stands alert, ready to assist,” Lower notes in The Ravine, the book she has written about the photo and her efforts to find the story behind it.Lower writes that during each interview she conducted with Miropol’s elderly Ukrainian citizens she showed them Škrovina’s photograph of the mother being shot. Did they recognize anyone? Every time, she says, when she “presented the photograph at the end of an interview, the subject looked and, with a shake of the head, turned away.”The photograph depicts only the instant of death. Soon neighbors will come to strip the corpses, carrying away clothes, gold teeth, and other valuables. They will cart away furniture from the Jews’ houses. Some will remember how the Germans came looking for Ukrainian volunteers; how the Jews screamed when they were marched off to the killing site; how the shots rang out for hours. Others will pretend that the Jews simply disappeared one day, and that the lives and deaths of Jews and Ukrainians had nothing at all to do with each other.Surprisingly, only a handful of photos from the Holocaust show Jews being murdered. There are many images of skeletal corpses in the death camps, emaciated prisoners, and humiliated ghetto Jews. But very few reveal the Nazis and their local collaborators pulling the trigger on their victims.There were four more photos in the portfolio that Lower saw that day in 2009, all of them showing the massacre in Miropol. Here was a visual record of the so-called Holocaust by bullets that reigned across Eastern Europe in 1941 and 1942. The most lethal two days of that slaughter were in Kiev on Sept. 29 and 30, 1941, when 33,771 Jews were murdered at the ravine of Babi Yar. Mass killing also occurred day after day at Ponary, near Vilna, and many other places in the Baltic states and Belarus, around the same time that the Nazis were trying to perfect a new, better means of extermination by poison gas.Lower traveled to Slovakia, Israel, and America looking for clues to the family in the photograph she found. While she still isn’t sure who they are (she has a hunch), she learned much about what happened in Miropol in October 1941. In 2014 and 2016 she interviewed 16 of Miropol’s Ukrainian citizens who remembered the October pogrom from their childhood 75 years earlier. Some recalled peeking through their windows at the Jews being rounded up; others, who were gathering wood in the forest, witnessed the Jews standing at the edge of the pit and being shot from behind.Lower tells us that two Miropol women born in 1926 “remembered the names of their Jewish classmates and friends who had invited them for Passover meals and treats.” Others recalled “playing hopscotch and hide-and-seek with their Jewish friends.” A few, she adds, “openly displayed their anti-Semitism, scoffing at the rich Jews and Bolshevik Jews from those days.”The biggest surprise in The Ravine is the identity of the photographer behind the image. “I had assumed that anyone who took such close-up action shots of a murder must be complicit,” Lower said in an interview. In fact the photographer, Lubomir Škrovina—a soldier in the Slovakian army, allied with Germany—was a member of an anti-Nazi resistance group from 1942 on. His photographs were intended to expose Nazi barbarity. Before he came to Miropol, he had taken snapshots of Jews lying in the streets covered in blood while German and Slovakian soldiers posed over them.Škrovina’s letters to his wife from the front are full of despair. “If I described to you some of the other grotesque images in my mind, you would be horrified,” he wrote to her in August 1941. “My thoughts are quite black.” After Škrovina returned home in December 1941, he hid Jews in his house and helped them escape to the woods. Škrovina was anti-Soviet as well as anti-Nazi, a man without an ideological home. Instead he had the capacity for humane response and, much more rare, the courage to act.Lower’s earlier book Hitler’s Furies, about women Nazis, explored the mystery of human motivation. The same person, she found, could both help Jews and murder them. In The Ravine there is no mystery. Lower simply describes the range of roles needed to kill a town’s Jews. Some will volunteer to commit murder, others will rape and hound the victims, one or two will shelter, at great personal risk, the tiny handful of Jews who manage to escape death for a few more days or weeks.Lower’s book is called The Ravine after the place where the remainder of Miropol’s Jews were murdered in early 1942. Those killed were “specialists” who were still needed by the Germans, like cobblers, carpenters, and the town dentist, months after the massacre depicted in the photo. Lower’s title also suggests the ravine at Babi Yar, the most notorious Ukrainian killing site. The ravine is an emblem, too, of the enormous absence that still haunts us.The Holocaust is the best-documented genocide in history, but vast gaps remain. Half of the Jews murdered in Ukraine, some 500,000 people, remain unidentified. Holocaust research is a losing race against time, as memories fade and death claims more witnesses every year. Soon there will be none left. So the detective work is urgent, but the chances of closing any given case are few. Every detail wrested from the past becomes a statement against the genocidal killers, and a way to honor the dead.Ludmilla Blekhman was, as far as we know, the only survivor of the Oct. 13 massacre in Miropol. She crawled out of the pit, from beneath the dead bodies of her family, ran into the woods, and found shelter with the Ukrainian forester. The 14-year-old girl spent more than two years, Lower writes, “living like an animal in the woods, hiding in fields and forests, fending off rats.” Blekhman was tortured by the Gestapo, and survived by swearing she was a Ukrainian peasant girl. In 1944 she returned to Miropol to see that her house had been physically torn apart by Ukrainian neighbors, who had used its wooden planks for firewood.Blekhman died in Israel before Lower could interview her, but her video testimony reveals many details of the Miropol pogrom. Blekhman recalled that the night before the massacre (as Lower writes),The Jewish elders gathered to devise a plan of escape. They determined that “the children must survive!” Each family was asked to identify a sympathetic and bribable Ukrainian who might hide the children. The adults would then attack the police and go down fighting. They attempted this during the Aktion, but nearly all their Ukrainian neighbors betrayed them.The Ravine is an elegantly structured book, as strange as it may be to use these words about a study of genocide. This compact book implies that the Miropol pogrom can stand for the whole of the Shoah, even though there is nothing here about gas chambers and sealed railroad cars. The mechanisms of mass death are secondary to the point of the genocide, as Lower sees it: the decimation of the Jewish family.The Nazis were intent, above all, on murdering Jewish women and children. In towns like Miropol, most adult Jewish men had fled to the East with the Red Army before the Nazis arrived. Imagining themselves to be useful to the Soviet war effort, they never thought that their families would be slaughtered in their absence. In similar circumstances, the great Yiddish novelist Chaim Grade left Vilna before the German assault, and returned after the war to find his wife and mother dead, as described in his memoir, My Mother’s Sabbath Days.The Germans did not waste bullets on children. They were most often smothered to death, covered in blood, dead bodies, and dirt. Lower notes that Jewish babies and toddlers in wartime Eastern Europe had less than a 1% chance of survival; Jewish children born after 1930 throughout Europe had less than a 2% chance. These are the lowest survival rates in history.Most Jewish children died alongside their parents or other relatives. At Treblinka mothers gripped the hands of their shaking, naked children, whose feet froze to the ground on the way to the gas chamber—the Himmelstrasse (path to heaven), the Nazis jokingly called it.“Nazi policy was two-pronged: family welfare and family destruction,” Lower notes. Nazi sentimental kitsch centered on the Aryan family, the wholesome pillar of the nation. But the Jewish family had to be exterminated. The Germans wanted to erase every vestige of Jewish memory, and children are memory bearers.There was another reason, too. In the case of the Jews, the most tender human bond, the one between mother and child, had to be denied. Jewish women and children were killed together without pity to prove that Jewish families had nothing at all in common with Aryan ones.The most heart-stopping of Lower’s chapters is “The Aktion,” which describes the pogrom in Miropol. First, the Germans commanded teenage Ukrainian peasant girls to dig a pit. Elderly Jewish men then continued the digging until the pit was one and a half meters deep, at which point they were shot and tossed into it. Then the Nazis recruited volunteers from the German customs guards in town to do the killing. Two men stepped forward, Erich Kuska and Hans Vogt.Twenty Ukrainian policemen sealed off Miropol to prevent Jews from escaping. Ukrainians then rounded up the Jews, clubbing and chasing them and raping the Jewish women. After spending the night in the marketplace, the Jews were led to the pit and murdered. Later, Lower writes, “The Ukrainian girls who had assisted as gravediggers were ordered to cover up the mass grave with soil and lime. The ground was moving.”Two years and two months after the murders in Miropol, Russia retook Ukraine, driving the German army back to the Polish border. The Red Army arrived in Miropol in January 1944, shortly after the bloody battle of Kiev. A number of the Nazis’ Ukrainian henchmen—the ones who had not fled or changed their identities—were executed on the spot. “A makeshift gallows was erected, and the police were noosed and made to stand on top of a truck, which then moved slowly away as a Soviet official pronounced the men ‘traitors to the homeland,’” Lower writes.During a Soviet trial in the 1980s headed by a KGB investigator, Mikola Makareyvych, three more Ukrainian killers were tried and sentenced, and two of them were given the death penalty. Erich Kuska, one of the German customs men who had volunteered to murder Jews, was interrogated in Bremen in 1969, but he denied his guilt, and the German police dropped the charges. The other German shooter, Hans Vogt, was never found.