A Los Alamos scientist gave the Soviets the secret to the hydrogen bomb five decades ago, allowing our arms race enemies to catch up during the tense early years of the Cold War, a pair of nuclear weapons veterans charge in a book being published next month.

The allegation, if confirmed by other historians, could answer one of the great mysteries of Cold War history: how the Soviets mastered the clever trick used by U.S. scientists to perfect the hydrogen bomb.

In their book, "The Nuclear Express," Thomas C. Reed and Danny B. Stillman say they believe they know who the spy was, and describe him in some detail.

But they decline to name the since-deceased scientist. "He can neither defend his family name nor refute our arguments," they write.

Reed is a former Lawrence Livermore National Laboratory nuclear weapons designer who served as secretary of the Air Force in the Carter administration. Stillman worked at Los Alamos National Laboratory from 1965 to 2001, and worked as the lab's intelligence director.

The U.S. had already tested several of the massively destructive weapons, and Soviet weapons designers were under pressure from the Kremlin in spring 1954 to duplicate the feat.

The central puzzle of the hydrogen bomb was how to create and then control enough energy to set off a nuclear fusion reaction -- the energy source that powers the sun.

U.S. scient ists, led by Edward Teller and Stanislaw Ulam, had come up with a scheme that used radiation from a first-stage nuclear blast to set off the fusion chain reaction.

Tested in November 1952 in the South Pacific, the Teller-Ulam scheme worked. The Soviets knew the test had been successful, but they had no idea how the U.S. scientists had done it.

The Soviets' effort to duplicate the feat the following summer was a dud, and it set off a frenzy of brainstorming by Soviet physicists, Reed and Stillman write.

In the end, the Soviets hit on the same approach as the U.S. weapons designers.

In the United States, the trick used to make the H-bomb work is forever known as the "Teller-Ulam design," and Reed said by telephone Wednesday from his California home that he was long puzzled by the fact that no Soviet scientist ever stepped forward to claim credit for the Soviet version.

The idea was elegantly simple, Reed said. A spy wanting to pass the U.S. secret to the Soviets would not have needed to share detailed blueprints.

To Soviet scientists already grappling with the problem, simply telling the Soviets the U.S. had used "radiation pressure" would have been enough to trigger the "aha" moment needed to design a working hydrogen bomb.

The allegation that a Los Alamos spy aided the Soviet H-bomb effort, first reported Tuesday by The New York Times, provides another chapter in the cloak-and-dagger tale of Soviet espionage in the early years of the U.S. nuclear weapons program.

In addition to the famous spies already documented in the history books -- David Greenglass, Klaus Fuchs, and Julius and Ethel Rosenberg -- some historians have argued that there was a mysterious spy, as yet unnamed, known by the code name Perseus.

Reed argued that Perseus ended his relationship with the Soviets in 1949, when Fuchs, Greenglass and the Rosenbergs were arrested.

But Reed and Stillman suggest that the Soviets, desperate to solve the riddle of the H-bomb, returned to Perseus for one last favor in 1954.

However they got the idea, by late 1955 the Soviets had built and tested their own H-bomb.

Reed acknowledged that he and Stillman have no proof of Perseus' role in sharing the secret, characterizing their finding as somewhere between "circumstantial evidence" and "a smoking gun."

If Stillman and Reed are right, they have uncovered one of the most important acts of espionage of the Cold War, Robert S. Norris, a leading nuclear historian, said in an interview.