By JAMES BAMFORD September 18, 2001 High in the frigid, crystalline air off the Kamchatka Peninsula of Siberia, a filter in a specially modified American B-29 began picking up traces of microscopic particles containing disintegrating nuclei. Like cancer cells in their earliest stages, the tiny bundles of atoms would portend devastating consequences. Scientists later determined that the invisible grains of matter caught in the plane's sniffer were highly radioactive and part of a cloud that was drifting east. Further analysis determined that they were produced by an explosion in a Russian desert about 100 miles south of Semipalatinsk. On Sept. 23, 1949, President Harry S. Truman announced to the nation the troubling discovery. The Soviet Union had successfully tested its first atomic weapon. The United States' short-lived nuclear monopoly was over, and the cold war suddenly shifted into high gear. Adding to the worry, the device appeared to bear a striking resemblance to the bomb the United States had dropped on Nagasaki four years earlier. Thus began the search for the mole who passed America's deepest atomic secrets to its mortal enemy. "The Brother," by Sam Roberts, takes a fresh look at the atomic- bomb spies Julius and Ethel Rosenberg from the perspective of the man who stole the secrets, and then gave up the Rosenbergs to the F.B.I. — David Greenglass. What makes the story especially poignant is that those whom Mr. Greenglass strapped into the electric chair as a result of his testimony were members of his own family — his sister and brother-in-law. Mr. Roberts, an editor at The New York Times, doggedly tracked down the elusive Mr. Greenglass in the early 1980's and pursued him for an interview, unsuccessfully, for 13 years. Then in 1996, on the verge of bankruptcy as a result of a failed business, Mr. Greenglass finally agreed to be questioned — for a share in the proceeds of the sale of the book. Thus he went full circle, first selling out his country for cash, then selling out his relatives for a deal with the prosecutors, and finally selling his story for a piece of a book. While Mr. Greenglass adds some personal detail and a bit of color, most of the key facts have long been known. He testified in court during the Rosenberg trial and gave a shorter interview to authors in the late 1970's. One exception, however, concerns Mr. Greenglass's testimony in which he confirmed a courtroom statement by his wife, Ruth, that Ethel typed up Mr. Greenglass's A- bomb notes. Now he says that he never actually remembered that happening. "I can only assume my wife didn't make it up," he said. But given the climate at the time — for example the jury only deliberated for 7 hours and 40 minutes — it is likely that the jury would have believed Ruth even without confirmation from Mr. Greenglass. Mr. Greenglass also adds little depth and insight into what J. Edgar Hoover called "the crime of the century." One keeps listening for Kim Philby but hears only Forrest Gump. Luckily, Mr. Roberts made up for the lackluster confession by doing a wonderful job of research. He went through box loads of yellowing archives and newly declassified memos. He interviewed nearly everyone still living who was connected with the case, and he uncovered numerous unpublished notes and manuscripts of key figures. The result is an important and highly readable tale of blind loyalty and bitter deceit, of hysteria and horror. Today's world of treachery is populated mostly by those who, out of need or greed, sell secrets to rescue a mortgage or buy a speedboat. But in the early 1940's ideology was a key reason for espionage, both in the United States and Britain. With the United States engaged in a world war, there was great uncertainty about who would eventually emerge as friend or foe. Few would have guessed that western Germany and Japan would become America's great allies, or that the United States would soon be looking down the barrel of our ally at that time, the Soviet Union. This led some Americans to sympathize with the Soviet Union's plight, facing Hitler on its doorstep, and with the utopian ideal of Communism. Julius and Ethel Rosenberg were among the few who went several steps beyond sympathy to espionage. True believers in Communism almost from puberty, they also indoctrinated Mr. Greenglass early on. Like many teenagers David had a paper route, only he delivered The Daily Worker. Bounced from college, Mr. Greenglass married a woman who shared his Communist sympathies and shortly thereafter, in 1943, was drafted into the Army. As chance would have it, he was assigned to Los Alamos and the Manhattan Project to develop the atomic bomb. For Mr. Rosenberg, by then deeply involved in espionage for the Soviets, it was a perfect opportunity, and he persuaded Mr. Greenglass to pass secrets to a courier, Harry Gold. The F.B.I. finally caught up with Mr. Greenglass in 1950, and he quickly agreed to trade a reduced sentence for himself and freedom for his wife for damning testimony against his sister and her husband. In a truly unusual move for a spy case, the Rosenbergs chose loyalty to the cause and silence instead of selling out anyone else. They were duly rewarded with the electric chair by a judge who maintained that God had told him to impose the death sentence. After 10 years, Mr. Greenglass walked out of prison with a new name and straight into anonymity, aided by the F.B.I. Eventually he and his family managed to retreat far enough into the background that even his grandchildren still do not know his true identity. "No. I still don't believe I did anything wrong," Mr. Greenglass replied when Mr. Roberts asked him about his espionage. "Can you imagine if there wasn't mutually assured destruction?" he said, single-handedly taking credit for keeping the world safe from nuclear destruction. "Would you ever say you're sorry to Ethel and Julius?," the author asked. "Never," Mr. Greenglass replied, adding that the Rosenbergs had an opportunity to cooperate with the government and foolishly decided against it. "To die for something as nebulous as that is stupidity," he said, quoting his and Ethel's mother. In the end, like Forrest Gump, Mr. Greenglass managed to encapsulate the entire case in a single phrase. "All you need is one guy to get caught," he said. "One guy that's not smart." Copyright 2001 The New York Times Company