A parliamentary probe into the state spy agency's alleged meddling in last year's presidential election began Wednesday with a hearing attended by justice ministry officials as rival parties questioned the legitimacy of a now-complete prosecution investigation into the scandal.

The parliamentary probe aims to uncover the truth behind allegations that former National Intelligence Service (NIS) chief Won Sei-hoon ordered an online smear campaign to sway public opinion in favor of the ruling Saenuri Party ahead of last December's presidential election.

Last month, prosecutors indicted Won and former Seoul metropolitan police chief Kim Yong-pan on charges of meddling in the election.

Kim has been accused of downscaling a police investigation into the scandal in its early stages.

The ruling party claims that the alleged smear campaign was in fact an attempt by the NIS to eliminate pro-North Korea opinions on the Internet ahead of the election.

"Was the NIS' psychological warfare staff using their status as public officials when posting anonymous comments on websites and clicking on 'recommend' or 'disapprove?'" Rep. Kim Do-eup of the ruling party said during the hearing at the National Assembly.

Justice Minister Hwang Kyo-ahn said not all of the NIS'

psychological warfare activities are classified as illegal, but the charges against Won were related to certain illegal elements that were found in the spy agency's activities.

Rep. Jung Chung-rai of the main opposition Democratic Party defended the prosecution's decision, claiming Won illegally intervened in the election and influenced the vote. (Yonhap News)