Prosecutors are looking into suspicions that those documents might have been forged and used to help him apply for graduate schools, including law schools. Cho's son is currently studying at a graduate school of Yonsei University.

Similar internship certificates were issued for Cho's 28-year-old daughter and a son of a Dankook University professor who helped list her as the primary writer of a medical paper while she was still in high school.

The prosecution is reportedly looking into suspicions that Cho might have been involved in the issuance of those internship documents.

Cho denied the allegations on Monday, saying that he is "seriously" thinking about taking legal action about what he called a malicious media report that raised doubts about the minister's role.

He said his daughter participated in the internship program in 2009 and the organization issued the related certificate.

Cho's wife was indicted on Sept. 6 for alleged forging a college presidential citation to help her daughter enroll in a medical school in the southern city of Busan.

The controversy over Cho's family has emerged as an explosive political issue that has sent the approval rate for President Moon Jae-in to a record low.

The ruling bloc and supporters of the minister have denounced the investigation as a politically charged move intended to hamper the government's efforts to reform the prosecution.

President Moon's appointment of Cho reflects his firm commitment to reforming the elite investigative agency in a way that promotes political neutrality. The justice minister oversees the prosecution.

In his first public comment on the probe, Prosecutor General Yoon Seok-youl said Wednesday the investigation is being conducted in an appropriate manner. (Yonhap)