A sitting prosecutor was questioned Tuesday over allegations of conspiring with the main opposition party to investigate key ruling camp officials last year when Prosecutor-General Yoon Seok-youl, now a leading opposition presidential contender, was in office.

Prosecutor Son Jun-sung appeared for questioning at the Corruption Investigation Office for High-ranking Officials (CIO) for allegedly asking Kim Woong, a prosecutor-turned-lawmaker of the main opposition People Power Party (PPP), to lodge a defamation accusation against the three figures, including Rhyu Si-min, a liberal commentator and former chairman of the Roh Moo-hyun Foundation, last year.

Son entered the CIO office in Gwacheon, south of Seoul, in a government vehicle and through a non-main entrance to avoid reporters.

The CIO had sought an arrest warrant for Son but a court denied the request last week, citing insufficient grounds for his arrest.

Investigators in charge of the case reportedly confirmed Son's involvement after a team analyzed a massive trove of digital data and documents obtained from a whistleblower and the Supreme Prosecutors Office.

Son has categorically denied all allegations. Investigators are reportedly scheduled to question Kim on Wednesday.

The CIO has also formally booked Yoon on multiple charges, including abuse of power, in its probe into the scandal.

Yoon has denied involvement in the case. (Yonhap)