MOSCOW -- Just minutes before his murder, Stanislav Markelov was at his most defiant. Protesting the release of a war criminal he had helped convict, he declared: "The person who decided to free him ... should be held criminally responsible."

As one of Russia's top human-rights lawyers -- a rare and endangered species -- Mr. Markelov had many enemies, from politicians to Nazi sympathizers. In January, someone took revenge. Mr. Markelov was walking along a busy Moscow sidewalk after his speech when a man shot him twice in the...