New secrets behind the greatest intelligence loss in US history.

The Soviet Union exploded its first atom bomb in 1949, years earlier than expected. But was their design based on nuclear secrets stolen from Los Alamos? Recently de-classified documents prove that hundreds of Americans spied for the former Soviet Union- yet only a few were prosecuted.

In the late 1940s, after American codebreakers successfully deciphered Soviet telegrams, the FBI nabbed Klaus Fuchs- as well as Julius Rosenberg- for giving atomic secrets to the Soviets. But only recently historians have discovered that as many as 300 Americans- including top-level Roosevelt Cabinet members- may have spied for the Soviets during World War II. And while the Rosenbergs were executed, a physicist named Ted Hall, who was identified early on by the FBI as one of the most important spies, got away without punishment.

Join NOVA and see how American perseverance and ingenuity cracked a Soviet coding technique believed to be impenetrable. Hear why many experts now believe the Rosenbergs played only a minor role in the loss of atomic secrets. Find out why the truth about high-level Soviet espionage was kept from the American public for nearly 50 years. And, for the first time, hear Ted Hall’s wife share his gripping story.