Hans-Joachim Klein is a former member of the German left-wing militant group Revolutionary Cells (RZ). In 1975 Klein participated in an attack on OPEC headquarters in Vienna organized by Carlos the Jackal, in which he was seriously injured. He publicly renounced political violence two years later. After decades in hiding, he was arrested in 1998 and prosecuted for his role in the OPEC attack. Klein was sentenced to nine years imprisonment and paroled in 2003.