In time, Wallenberg’s rescue work became an lasting symbol of the international human rights movement, but the mystery of his fate seemed likely to endure forever — until the Serov diaries came to light.

Memoirs from high-ranking Kremlin officials are exceedingly rare, and this one, while hardly definitive, contains several references to previously unknown documents on Wallenberg.

They include a report about Wallenberg’s cremation, and another quoting Viktor Abakumov, who preceded Serov as head of state security but was tried and executed in 1954 in the last Stalin purges. Abakumov apparently revealed during his interrogation that the order to “liquidate” Wallenberg had come from Stalin and Vyacheslav M. Molotov, the foreign minister.

The word “killed” has never appeared in any official documents released from the Soviet side, according to Nikita Petrov, a historian with the Memorial organization in Moscow who specializes in the Stalinist era and Serov himself.

Image Raoul Wallenberg, a Swedish diplomat and World War II hero who disappeared in 1945. Credit... Pressens Bild/Associated Press

“They did not use this word,” Mr. Petrov said. “They said it appears he was killed, but we know nothing about this, we don’t have any documents. In Serov’s diary, you can find this word as a fact.”

Memoirs lack the weight of official documents, Mr. Petrov noted, but Serov also described reading a Wallenberg file. Previously, the security service denied that any such files existed, according to diplomats, historians and others who have long worked on the case.