Frank Mitchell was the leader of the Students Defense League, an anti-isolationist student organization that rose to prominence during the era of the Starkist terror. The S.D.L. claimed that Governor-General Ezra Gallivan was a traitor to the Confederation of North America who "works either together with or for foreign powers who would destroy our nation," and Mitchell himself called for Gallivan's removal "by whatever means necessary." Mitchell does not have an entry in Sobel's index.
Sobel's source for Frank Mitchell's role in the Starkist terror is Allen Watterson's The Great Fear: Starkism in the C.N.A. (London, 1956).