In the 1970s Klein worked for the legal Red Army Faction prisoner support group Red Aid ("Rote Hilfe") in Frankfurt and subsequently for the RAF lawyer Klaus Croissant in Stuttgart. At one point he shared a Frankfurt commune with Joschka Fischer and Daniel Cohn-Bendit. He served as Jean-Paul Sartre's chauffeur when Sartre visited the imprisoned Andreas Baader in December 1974. He joined the RZ in 1975. Klein has described Holger Meins' death on hunger strike as inspiring his turn to violence; he carried a photo of Meins' emaciated body in his wallet. He took part in the 1975 attack organized by Carlos the Jackal on an OPEC conference in Vienna, in which three people were killed and Klein himself shot in the stomach.