The Seoul Prosecutors' office plans to summon Korean-German professor Song Doo-yul Friday for a fourth round of questioning on suspicions that the dissident scholar had violated South Korea's National Security Law.

Unable to gather any conclusive evidence in the previous three bouts of questioning, the prosecution said that it would summon Song once again to investigate the charges against him, calling the scope of the investigation 'enormous'.

Prosecutors are reported to have confirmed most of the charges against Song to be true after questioning related figures including Hwang Jang-yup, a defector and former secretary of the North Korean Workers' Party, and Oh Kil Nam, a former professor in Germany.

The prosecution has said that it would exert leniency in any future criminal punishment against Song if he actively cooperates during the questioning and demonstrates remorse for his past anti-South Korean activities.