SEOUL, South Korea — Agents from the National Intelligence Service of South Korea posted more than 1.2 million Twitter messages last year to try to sway public opinion in favor of Park Geun-hye, then a presidential candidate, and her party ahead of elections in 2012, state prosecutors said on Thursday.
For months, South Korean politics have been rocked by the opposition’s accusations that officials at the National Intelligence Service, the country’s spy agency, and the military conducted an ambitious but clandestine online campaign to help Ms. Park win the Dec. 19 election.
Prosecutors have indicted several top intelligence officials, including
Won Sei-hoon, the former director of the spy agency, on charges of ordering an online smear campaign against opposition candidates in violation of election law. A team of agents posted online messages that lauded government policies while ridiculing Ms. Park’s opposition rivals as untrustworthy pro-North Korean sympathizers ahead of the parliamentary election in April last year and the subsequent presidential election, they said.
But the prosecutors could not clarify how the operation affected the result of the elections. Ms. Park, who won her election by one million votes, has said she neither ordered nor benefited from such a campaign. But the opposition party claimed that she and the conservative government of her predecessor, Lee Myung-bak, colluded to manipulate the election results.
The evidence unveiled by prosecutors on Thursday showed that the Twitter campaign was more expansive than previously known. The revelation came as political pressure has mounted on the prosecutors. In the National Assembly, the opposition is pushing for the appointment of an independent investigator, saying that the investigation by the prosecutors cannot be trusted.
During a budget speech to the National Assembly on Monday, Ms. Park lamented the prolonged political strife, which has grounded many economic and tax overhaul bills. She promised to block the intelligence agency from meddling in domestic politics but called for people to trust the prosecutors and the court to investigate the election scandal.
On Thursday, her deputy, Prime Minister Chung Hong-won, said the prosecutors’ new findings were evidence that they were being fair. The governing Saenuri Party also accused the opposition of starting a political offensive to discredit Ms. Park’s legitimacy as president.
“We don’t think that the prosecutors’ fairness and neutrality were compromised,” Yoon Sang-hyun, a deputy floor leader, was quoted by his party as saying during its leadership meeting.
But the main opposition Democratic Party called for the dismissal of Justice Minister Hwang Kyo-ahn, accusing him of soft-pedaling the prosecutors’ inquiry to prevent any finding that would hurt Ms. Park. Mr. Hwang, appointed by Ms. Park, oversees the prosecutors. Opposition party leaders have also held a series of rallies in recent months demanding an apology from Ms. Park, whom they accuse of obstructing a fair investigation.
“What’s clear so far is that the National Intelligence Service and other state agencies had engaged in a systematic and massive intervention in elections,” Kim Han-gil, the top opposition party leader, said on Thursday.
The intelligence service said its online messages were posted as part of normal psychological warfare operations against North Korea, which it said used the Internet to criticize South Korean government policies, forcing its agents to defend them online. In a statement on Thursday, it also accused the prosecutors of citing as their evidence online postings that had nothing to do with its agents.
The allegation first surfaced during the election campaign last year, when opposition politicians and officials from the National Election Commission tried in vain to enter an office in Seoul that was locked from the inside by an intelligence agent who refused to answer questions about whether she was part of an illegal online election effort.
Three days before the presidential election, the Seoul police announced that they had found no evidence to support the opposition’s accusations. During her last television debate, Ms. Park excoriated her main opposition rival, Moon Jae-in, over what she called the harassment of a female agent by his party.
But the scandal did not die with Ms. Park’s election.
A senior police investigator told reporters after the election that her supervisor had intervened in the investigation, withholding evidence. The boss, Kim Yang-pan, who is the former chief of the Seoul Metropolitan Police, was indicted with Mr. Won, the former intelligence chief. Both denied the charges against them.
At the time of Mr. Won’s indictment in June, the prosecutors said they had found thousands of online political postings by his agents since 2009. Then, last month, they said they had found more than 55,000 Twitter messages from them. The former head of the prosecutors’ investigation also said his boss in the Seoul Central District Prosecutors’ Office tried to block him from submitting that additional evidence, a charge the boss denied.
In a separate inquiry, military investigators are looking into South Korea’s Cyberwarfare Command after it was revealed last month in Parliament that some of its officials had conducted a similar online campaign against opposition candidates. The Cyberwarfare Command was created in 2010 to guard South Korea against hacking threats from North Korea.
On Thursday, the prosecutors said the 1.2 million Twitter messages they had discovered were mostly copies of the 26,500 original messages that the agents mass-distributed through a special computer program. But even if they were copies, they constituted an act of meddling in domestic politics and elections, Lee Jin-han, a senior prosecutor, told reporters.