He is the third former NCA chief to undergo prosecution questioning over suspicion that top court officials worked at the behest of then-Supreme Court Chief Justice Yang Sung-tae to interfere in trials in order to lobby the presidential office of then-President Park Geun-hye for Yang's pet project.

Prosecutors have questioned Cha Han-sung and Park Byong-dae, the other two ex-NCA chiefs, over the allegations and arrested former NCA deputy head Lim Jong-hun for being the key man in carrying out Yang's schemes.

Ko faces charges of abuse of power and obstruction of justice, among others.

Ko is accused of covering up an appeals court judge's wrongdoing in the southeastern city of Busan in 2016 after he found out that the judge, surnamed Moon, tried to leak information about the bribery trial of a businessman. Moon had apparently been receiving kickbacks from the businessman.

Prosecutors suspect that Ko used his influence to overlook Moon's wrongdoing because Moon had close ties with Hyun Ki-hwan, then the presidential secretary for political affairs.

Yang needed their connection to lobby the presidential office for Park's approval to set up a separate court of appeals, his long-envisioned goal, the prosecution has alleged.

Ko is also suspected of unfairly influencing a trial involving a progressive teachers' union with the intention of suppressing them in line with the Park government's policy. (Yonhap)