SEOUL, South Korea — State prosecutors said on Tuesday that they would indict a former chief of the South Korean National Intelligence Service on charges of illegally trying to influence the election in December in favor of Park Geun-hye, who was elected president as the governing party candidate.
The former official, Won Sei-hoon, served as the top intelligence official under President Lee Myung-bak, Ms. Park’s predecessor, until February. He is accused of being involved in a secretive campaign in which intelligence agents and bloggers hired by the agency posted negative comments on the Internet about Ms. Park’s rivals.
No evidence has emerged that Ms. Park — who defeated her main opposition rival, Moon Jae-in, by one million votes — was involved in the operation. It also remained unclear whether or how much the online campaign, if it existed, influenced the result of the election. But the announcement added weight to a long-held suspicion among the political opposition that the intelligence agency engaged in illegal campaigning for Ms. Park.
Mr. Won will be charged with violating the national election law, which bans government officials from using their influence to affect an election, as well as a separate law that prohibits government intelligence officials from meddling in domestic politics, Lee Jin-han, a senior prosecutor, told the South Korean news media on Tuesday.
Mr. Won, who was not arrested, was not immediately available for comment. But he and the intelligence agency have denied interfering in the election on Dec. 19, saying the office’s online activities were part of normal psychological operations focused on North Korea.
Ms. Park’s office did not comment on the prosecutor’s announcement on Tuesday. During her campaign, Ms. Park and her party accused Mr. Moon and his opposition party of stirring up trouble when they first made accusations of illegal campaign activities by intelligence agents.
Mr. Moon’s opposition Democratic Party on Monday accused Ms. Park’s office and her justice minister of trying to intervene in the prosecutors’ investigation. Both the presidential office and prosecutors denied the claim.
Mr. Won is the latest in a series of former South Korean intelligence chiefs who have faced criminal indictments after leaving office. Several of them have been imprisoned.
The head of the National Intelligence Service, once used by the country’s military dictators as a main tool for silencing political dissent, remains one of the most powerful government jobs in South Korea. The intelligence agency has repeatedly vowed not to meddle in politics.
Police officials, who rejected the opposition’s accusation before the election, reversed their position in March, announcing that two low-ranking agents had illegally posted comments online criticizing the opposition candidates.
Prosecutors took over the investigation in April, as the opposition accused the police of whitewashing their inquiry so as not to offend Ms. Park.
Mr. Lee, the senior prosecutor, also said on Tuesday that the former head of the Seoul police, Kim Yong-pan, would be indicted on charges of illegally intervening in the police investigation.
The announcement of the charges came after prosecutors summoned Mr. Won and other senior intelligence officials for questioning and raided the spy agency’sheadquarters in a Seoul suburb in April.