The Seoul Prosecutors' Office questioned two people connected to the Korean-German sociology professor Song Doo Yul Tuesday, searching for clues related to the allegations of pro-North Korean activities swirling around the former dissident.

The prosecution said that it summoned two witnesses for questioning, but did not reveal their identities, as the witnesses sought to remain anonymous.

The prosecution's investigation was reportedly focused on clarifying questions over Song's alleged status as a Politburo member of the North Korean Workers' Party.

Prosecutors said the investigation is progressing slowly, and that it might take considerable time to make a final decision on whether to press criminal charges against the professor.