The body of West German Federal Prosecutor Siegfried Buback and his driver Wolfgang Goebel, right, lie covered on the street in 1977 after their murders by members of the Red Army Faction
Public broadcaster Suedwestrundfunk said they had found Buback's Nazi party membership card, after a witness in a trial over his murder revealed the information last week.
Stefan Wisniewski, a former terrorist from the left-wing Red Army Faction (RAF), made the information public as he testified in court at the trial of fellow extremist Verena Becker over her alleged role in Buback's assassination.
Several RAF members were collectively convicted of murdering Buback and two of his bodyguards, but they never revealed who fired the deadly shots.
Buback's Nazi past was not publicly known - and in all likelihood would not have been known to the RAF terrorists who rebelled in the 1970s against the perceived suppression of Germany's Nazi history.
The Nazi membership files were only opened to the public in 1990. Yet state authorities, including the prosecution agency headed by Buback, had always been aware of his wartime allegiance, an agency spokesman said.
Becker, now 58, is accused as a joint perpetrator in Buback's murder, but only for her role in planning and organizing the assassination.
Wisniewski is also being investigated for murder after former RAF members indicated that he fired the shot, but there have been no conclusive leads.