MOSCOW — A court in Moscow will hear a lawsuit filed against Russia’s top security agency by relatives of Raoul Wallenberg, the Swedish diplomat who saved thousands of Hungarian Jews from death in Nazi extermination camps, a lawyer for the family said on Thursday.

Mr. Wallenberg’s relatives began fighting decades ago to learn more about what happened from the time of his disappearance from a Budapest street in 1945 to his mysterious death, supposedly in 1947, in a notorious Soviet prison.

Marie Dupuy, Mr. Wallenberg’s niece, is seeking to force the F.S.B. security agency, a successor to the K.G.B., to make public documents that could shed light on his fate. She believes the documents are in the agency’s archive.

During the final stages of World War II, Mr. Wallenberg served as Sweden’s special envoy in Budapest and courageously used his diplomatic privileges to prevent Jews from being deported to concentration camps.