The Whip Hand (1951)

Matt Corbin, a vacationing magazine writer, takes a fishing trip to Minnesota, and stumbles across a lake in which all the fish have mysteriously died. The locals are tight-lipped about it, but Corbin learns that a group of former-Nazis-turned-Communists have purchased a lodge on an island in the middle of the fish-killing lake, and have built some kind of laboratory. Never one to pass up a chance to sell a story to a magazine, Matt decides to investigate. His only ally is Janet Keller, the sister of the local doctor who has been caught up in whatever those nefarious Commie-Nazis are up to. What they are up to, with Soviet financing, is the development of diseases to use in bacteriological warfare against the United States, starting right there in Minnesota.