More than 100 American soldiers — all of them quite elderly now — who liberated Nazi concentration camps at the end of World War II gathered in Washington, DC this week. They were there to commemorate the 20th anniversary of the US Holocaust Memorial Museum. The veterans met survivors of the Nazi genocide and shared their stories. The World's Middle East Correspondent Matthew Bell recently met someone with a unique Holocaust story that he was somewhat reluctant to talk about. It's a story about revenge. The Holocaust has become so important to Israel's identity, it seems hardly a day goes by in this country without some reference to the Shoah — or catastrophe — as it's known in Hebrew. Students at a Jerusalem high school took part in their annual ceremony on Holocaust Memorial Day in early April. This is where I met Yossi Cohen. He's an 87-year-old Holocaust survivor, born in Lithuania. He was at the school to speak with kids about the Holocaust. But he didn't share his whole story. Because Yossi Cohen — not his real name, by the way — was part of a "Nakam" — or vengeance — group in post-war Europe. Even after Germany surrendered, its members were still plotting to kill Germans as an act of retribution for the deaths of millions of Jews. At his apartment in a posh section of Tel Aviv, Cohen shows me an old photo taken near the end of the war. He's in uniform: A wool Lithuanian jacket, German cavalry officer's trousers, knee-high Russian boots, and a German army belt, with an inscription on the silver buckle. The inscription reads 'Gott mitt uns,' which means 'God is with us.' And there's a hole in the belt buckle. "So, in this case, God was not with him, because I took the belt," Cohen said. He took the belt, but when I asked if he took the shot that killed the German soldier wearing it, Cohen would not say. He wants to be careful talking about his past. Cohen would only agree to an interview if we didn't use his real name. But when we sat down in his living room, he seemed keen to talk about the war. For him, it began suddenly in the summer of 1941. He had just graduated from high school in Vilnius, Lithuania. And that evening, he went a party and did some dancing. Who did you dance with on the night that you graduated? I asked. "One of my teachers," he said. "A young teacher, very good-looking. And it was an occasion to dance with her." The next day, Cohen heard the sounds of war. Germany began its invasion of the Soviet Union. In Nazi-occupied Lithuania, the mass killing of Jews started right away. Cohen and his family learned how to hide. With his parents and younger brother, Cohen moved between safe houses for a few months. Then, they entered the Jewish ghetto in Vilnius. He was 17. And naturally, Cohen said, he was eager to join the underground, the resistance. "So, I was looking for somebody who would give me some kind of contact," Cohen said. I asked him why he wanted to join the underground. "To do something," he said. "To fight. Just to go, be taken and be killed? I mean, a young man?" Cohen eventually joined the Jewish resistance. Helped collect weapons. He also taught people how to shoot. And he became a commander. When his mother found out, she asked him not to recruit his little brother. "Leave me one son," she told him. But when the ghetto was liquidated a year later, Cohen was alone. His father had disappeared. His brother was sent to a concentration camp. His mother was sent off to the camps as well. Cohen escaped the ghetto and helped lead Partisan fighters living in the forest. I asked what he was feeling, did he have a feeling of, 'vengeance'? "Very much," he said. "Very strong. Very strong. We could do a lot. What did you want to do then? I asked. "We did everything," Cohen said. And by 'everything,' Cohen means attacking German soldiers. Blowing up supply trains and burning bridges, sabotaging rail lines with help from the Soviet army. But he said the desire for vengeance didn't go away, even after the defeat of Nazi Germany. In fact, it grew stronger for Cohen and some of his comrades as they learned the true scope of the Holocaust. "We saw that something has to be done," he said. "You cannot pass on to the history without doing something." Cohen said there needed to be some kind of reckoning. But here's where he gets stingy with details. In 1946, Jewish revenge groups hatched a plan to poison the water supply in German cities. The aim was to kill Germans on the same scale that they killed Jews. Millions of them. Cohen would not say if he was involved, though plenty of people volunteered he said. But many Jewish leaders wanted strong, activist Jews to move to Palestine rather than focus on retribution. "The Israeli Jews were not there," Cohen said. "And nobody who wasn't there and didn't live all these disasters couldn't think like us." British writer Jonathan Freedland has researched Jewish revenge groups and he wrote a novel based on them called "The Final Reckoning." Freedland said there's a quixotic element to this chapter of Jewish history. But that doesn't diminish the profound moral outrage that motivated people like Yossi Cohen. "I think they would have spoken for hundreds of thousands — if not millions of Jews — at that time, who didn't know what they were doing in their name, but nevertheless felt that same sense of outrage that this crime, which I think few people would dispute a description as the greatest crime in human history, this crime had gone fundamentally unpunished," Freedland said. The plot to poison the water supply in German cities was ultimately foiled. But there was another incident, Cohen said. It was at an American-run prison for German soldiers, known as 'Stalag 13.' "Stalag 13 happened," Cohen said. I asked if people were poisoned and if he knew who did it? "Yes," Cohen said with a laugh. When I asked Cohen if he was involved, he won't say. But here's how the story goes. A group of Jewish avengers found out where the bread was made for a prison holding German SS officers. They managed to infiltrate the bakery and poison one morning's batch of bread with arsenic. Jonathan Freedland said an Associated Press report from April 1946 said 1,900 German POWs fell ill. "It's still never been verified how many SS men actually did die at Stalag 13," Freedland said. "Some say it was in the low 100s. Some people have put the figures as high as one thousand. But that was a very direct form of vengeance. These were SS men who had themselves perpetrated or participated in the murder of Jews and they were themselves killed. That was really the biggest ever vengeance operation in history. And it was certainly the biggest ever pulled off by this group." Before the start of the Holocaust memorial ceremony at the high school, I asked Yossi Cohen if the students knew about his involvement with the revenge group. Probably not, he told me. Cohen is not sure they'd understand what the group did after the war. But he said he doesn't regret anything. There needed to be proper revenge, he said. Normal people had to do something to make sure nothing like this ever happened again.