Wrapping up a three-month probe, a special prosecutor team failed Thursday to disclose what drove a cyber attack on the National Election Commission's Web site last year, despite suspicions the ruling Saenuri Party tried to influence the results of the October by-elections.

The special prosecution investigation came after opposition parties voiced doubts about the results of an initial probe into the case, which found a parliamentary aide responsible for the so-called distributed denial-of-service (DDoS) attacks on election day on Oct. 26.

The aide had worked for then Saenuri lawmaker Choi Ku-sik, leading to allegations the ruling party masterminded the attack to block a rival candidate from winning the mayorship of Seoul.

Park Tae-seok, a prosecutor on the special investigation team, told a press conference the prosecution indicted five people in connection with the case, although none were held directly responsible.

Kim Hyo-jae, a former senior secretary for political affairs at the presidential office, was indicted on charges of leaking sensitive information related to the initial prosecution investigation last year.

Kim allegedly informed then-lawmaker Choi on Dec. 1 that his aide surnamed Kong and three others had been arrested in connection with the hacking probe.

Two other officials at the presidential office were indicted on similar charges, along with an employee of the election commission and an employee of a telecommunications company, Park said. (Yonhap News)