The 1945 disappearance of Raoul Wallenberg — a Swedish diplomat who saved thousands of Hungarian Jews from Nazi gas chambers — ranks among the most enduring mysteries of World War II.

Suspicion for the snatching of Wallenberg off the streets near Budapest fell almost immediately on the Soviet Union. To the Soviets occupying Budapest, the ties that Wallenberg had forged with senior Nazis and Americans smelled like espionage, with rescuing Jews an implausible cover story. But his disappearance went unexplained, right through the Gorbachev era of glasnost and the chaos after the collapse of the Soviet Union.

This summer, however, the newly published diaries of the original head of the K.G.B. — found secreted inside the wall of a dacha — have shed fresh light on the case by stating outright for the first time that Wallenberg was executed in a Moscow prison.