Alibi By Joseph Kanon Henry Holt 405pp., $26 Though it was a refuge for dowagers, painters, poets and the idle rich, Venice could not be a refuge for its Jews during World War II. After Italy signed an armistice with Germany in September 1943, roundups began in northern Italian cities. On November 30, G. Buffarini Guidi, Mussolini's interior minister, ordered all Jews to be sent to camps. Even though Italy was not as culturally Jewish as other parts of Europe - there had been more Jews in Krakow than in all of Italy, for example - there must have been an uneasy feeling in Venice once the war was over. The word ghetto, after all, originated there, and Jews were an important part of the community. Nazi collaborators must have been visible on the street or at the opera house after the war. What would their alibi be? Would anyone even ask them? Here are the questions animating Joseph Kanon's fourth moral thriller, Alibi, a novel full of shades of gray. Our guide through this labyrinth of truths and half-truths is Adam Miller, a 20-something American G.I. who spent his tenure in the Army hunting down Nazi war criminals. Adam has come to Venice to rest and recuperate and to visit his rich widowed mother, who has come to recapture the golden days of her youth. The clash of these two purposes gives Alibi an unusually intimate kind of tension, one that ratchets higher when Adam meets Claudia, a Jew who escaped death by acting as mistress to an Italian fascist. She has returned to Venice and lives a determinedly low-profile life... until Adam brings her to a party and introduces her to his mother's new beau, Gianni Maglione. All at once, Claudia recognizes the Venetian doctor who turned her father over to the SS. This is Kanon's third book to deal with the postwar period, and it seems the most self-consciously literary of the group. Los Alamos unfolded in the spring of 1945 and involved the murder of someone attached to the Manhattan Project. The Good German evoked the wreckage of postwar Berlin through the eyes of a former CBS correspondent. Each of those books arose from its context with little apparent effort. In Alibi, however, you can feel Kanon working to tease mystery and furtiveness out of his Venetian setting. He wants us to see Venice of the expatriate community, not of the locals (as Donna Leon does in her Commissario Brunetti series). Kanon also wants us to appreciate the decadence of Venice's beauty, which of course the war spared. So Adam lingers a bit too long on the accoutrements of tacky money and tourist vistas: the torches set up for nighttime parties, the drinks every day at 5. Dialogue will occasionally halt so Kanon - via Adam - can pan meaningfully across the Piazza San Marco and show us all the beauty that watched over the Holocaust from afar, with a neutrality Adam views as criminal. As a result of all this excess prose, Alibi lumbers like a sprinter in a fat suit. There's a speedier creature in there, but Kanon just can't quite let it out. Once a murder actually occurs - some 150 pages into the book - the action speeds up, and Adam, Claudia, an Italian policeman and a former partisan named Rosa begin parsing the alibis of those involved. All this over a killing that during the war was not just OK but honorable. "It's a good lie," says one character about partisan revenge killings. "To kill. And then [the war is] over and it's the opposite." Kanon is right to examine the moral slipperiness of killing in and out of war, but these discussions sit heavily on the book, especially since Alibi takes so long to get started. To finish the book one needs to rush before the sunset of attention arrives. And as anyone who has visited Venice before will tell you, that's no way to spend time in a city that has stood the test of time so well.