We knew that

The New York Times has an article by William Broad that confirms the suspicion everyone's had for years:

A defining moment of the cold war came in 1955 when Moscow detonated its first hydrogen bomb — a weapon roughly a thousand times more powerful than atom bombs and ideal for obliterating large cities. The bomb ended the American monopoly and posed a lethal danger. So Washington dealt far more gingerly with Moscow, beginning a tense era dominated by fear of mutual annihilation.
Now, a new book says Moscow acquired the secret of the hydrogen bomb not from its own scientists but from an atomic spy at the Los Alamos weapons lab in New Mexico. Historians call its case sketchy but worthy of investigation, saying the book, The Nuclear Express: A Political History of the Bomb and its Proliferation, by Thomas C. Reed and Danny B. Stillman, adds to a growing number of riddles about who invented the Soviet H-bomb a half century ago.
“It’s quite intriguing,” Robert Norris, a nuclear historian, said of the book. “We’ve learned a lot about atomic spies. Now, we find out that a spy may be at the center of the H-bomb story, too.” A surprising clue the authors cite is disagreement among Russian nuclear scientists over who deserves credit for the advance as well as some claims that espionage played a role. The book details this Russian clash and questions the popular idea that Andrei Sakharov, who later became known as a campaigner for human rights, independently devised the Soviet hydrogen bomb. The book does not name the suspected spy but says he was born in the United States, grew up in a foreign country, fell in with communist sympathizers during the depression, and worked at Los Alamos during World War II. Afterward, it says, he became “deeply involved” in the American effort to develop the H-bomb.
The book says that Mr. Stillman, a physicist who worked at Los Alamos from 1965 to 2000 and served for more than a decade as the lab’s director of intelligence, took his suspicions in the 1990s to the Federal Bureau of Investigation. But the FBI inquiry, the book says, was “botched beyond recognition” and went nowhere. The alleged spy, the book adds, is now dead. The FBI, often accused of disarray in cases of atomic spying, declined to comment.
Historians and nuclear scientists call the book’s claim provocative, if vague and seemingly circumstantial. They add that its suspect is unlikely to be the last put forward to account for the Soviet breakthrough. “It’s a fascinating puzzle,” said David Holloway, author of Stalin and the Bomb and a military historian at Stanford University. “Mystery is too strong a word. But exactly how the Soviet physicists hit on the idea remains unclear.”
Harold Agnew, who worked on the world’s first H-bomb and eventually became director of Los Alamos, said the Soviets probably had had numerous spies divulging the secret. “We were always surprised,” he said, “at how quickly they moved ahead.”
The new book is due out in January from Zenith Press. A main focus is how spies spread nuclear secrets around the globe. In recent years, the ranks of known Soviet spies in the Manhattan Project to build the atom bomb have swollen to a half dozen or so, and more are expected to be named. But so far, accounts of the ensuing project at Los Alamos to build the hydrogen bomb have documented no major episodes of atomic spying. Hydrogen bombs, unlike their atomic cousins, are unlimited in size. American scientists who sought to devise one in the 1940s and early 1950s thus called their dream weapon the Super. The successful architects were Edward Teller and Stanislaw Ulam. Their 1951 breakthrough, known as “radiation implosion", called for putting an atom bomb at one end of a metal casing and hydrogen fuel at the other. The flash of the exploding atom bomb was to flood the case’s interior with enough radiation to compress and ignite the hydrogen fuel, releasing huge bursts of energy through nuclear fusion.
In late 1952, the first test of their idea caused the Pacific island of Elugelab to vanish. The explosion was 700 times more powerful than the blast that leveled Hiroshima.
Moscow had nothing comparable until 1955. It then made an arsenal of H-bombs that in time dwarfed Washington’s. It also detonated the world’s largest bomb — a behemoth more than 3,000 times as powerful as the Hiroshima blast. Over the decades, scholars identified Klaus Fuchs as one possible source of H-bomb intelligence. The Soviet spy in the Manhattan project left Los Alamos in 1946, gave Moscow H-bomb ideas, and was arrested in 1950. But most scholars judge his tips as too early, too sketchy and too erroneous to have provided much assistance.
The authors of The Nuclear Express said in interviews that their interest in the issue stirred after the cold war as former Soviet nuclear scientists told of their hidden labors. Mr. Reed, a former designer of H-bombs at the Livermore weapons laboratory in California and a former secretary of the Air Force, met a number of the Russian scientists at Livermore in March 1997. He said the meetings had proved eye opening. The Russian scientists described how Dr. Sakharov never took full credit for the hydrogen advance. And Lev Feoktistov, a member of the founding H-bomb team, suggested that espionage unrelated to Fuchs played a role. In his 1999 book, Nukes Are Not Forever, he reiterated that claim. “I cannot escape the feeling,” Dr. Feoktistov wrote, “that we were extended a helping hand once in a while, although quite inconspicuously.” For instance, he said the Soviet team had been given an unfamiliar bomb sketch that he subsequently identified as having been the work of Ulam, the American H-bomb pioneer. The sketch showed a design that antedated the breakthrough of radiation implosion. Amid the revelations after the cold war, Mr. Stillman, at Los Alamos, zeroed in on a candidate spy. In an interview, he said his suspicions had been aroused for a number of reasons, including the man’s great apparent wealth. Mr. Stillman said the FBI inquiry fell apart in the 1990s as the bureau’s Santa Fe office became entangled in the case of a modern alleged spy at Los Alamos: Wen Ho Lee. In time, all but one of the charges against Dr. Lee were dropped after a judge found significant flaws in the government’s case. The episode is seen as having raised the federal bar on new claims of atomic spying.
When Mr. Reed and Mr. Stillman began to collaborate on their book, they judged that they had complementary pieces of the H-bomb puzzle. In the book, they say they declined to name the Los Alamos suspect because he is now dead and “can neither defend his family name nor refute our arguments”. The actual identity does not matter, the books adds. “His fingerprints are what count.”
Reactions to the claim range from strong interest, to outrage, to curiosity about the identity of the alleged spy. For years, most Russian scientists and officials have insisted that the Soviet invention was completely independent of the United States, with the exception of preliminary intelligence from Klaus Fuchs. Gennady Gorelik, a Russian historian of science now at Boston University and a Sakharov biographer, dismissed the idea that the Soviets had received the secret from newly disclosed espionage. “No, they did not,” he wrote in an e-mail message.
Priscilla McMillan, an atom historian at Harvard and author of The Ruin of J. Robert Oppenheimer, said her weighing of old and new evidence had come down on Dr. Sakharov’s side as the main inventor. “It’s a tantalizing subject,” she said. “But I wouldn’t preclude that his version is pretty much correct.”
John Earl Haynes, a Library of Congress historian and an authority on atomic spying, said the book’s authors might have found a new spy at Los Alamos but he doubted their identification of him as a KGB asset. If the spy existed, he added, he might have been controlled by the GRU, a military intelligence agency. Richard Garwin, a top nuclear physicist who helped invent the American H-bomb and has advised Washington for decades, echoed Dr. Agnew in saying he found quite reasonable the idea that Moscow had espionage tips from Los Alamos about radiation implosion. “It is difficult to believe that US security was so good that the Russians could not have picked up the term,” he said in an interview. Dr. Norris, author of Racing for the Bomb, an account of the Manhattan Project, said solving the H-bomb riddle awaited more candor from Moscow. “The only way of clearing this up is for the intelligence services, the successors to the KGB and the GRU, to claim their share of the credit,” he said. But he added that such openness could undermine Russian pride in its nuclear achievements during the cold war. “It cuts both ways,” he said. “It would really be a blow to the self image of the Russian scientists, who believe to this day that they invented it independently.”

Rico says awww, bummer; the Russian scientists are just gonna have to get over it...