South Korea's National Intelligence Service (NIS) has been accused of using Twitter to sway public opinion ahead of last year's presidential and parliamentary elections, as part of an ambitious campaign to support Park Geun-hye, who won the presidency by a margin of one million votes last December. Accusations of foul play had been flying for months, but federal prosecutors last week unveiled new evidence detailing the scope of the operation.

According to the prosecutors, a team of NIS agents posted more than 1.2 million tweets that either praised Park and her policies, or smeared her rivals as North Korea sympathizers. The 1.2 million tweets unveiled last week were largely duplicates of 26,550 original tweets that were distributed through a computer program, pointing to what prosecutors describe as widespread meddling on the part of the NIS.

It's not clear whether the campaign had any influence over the election results, though opposition leaders have accused Park and outgoing conservative President Lee Myung-bak of orchestrating the operation to tilt last year's presidential and parliamentary elections in their favor.

"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," opposition party leader Kim Han-gil said last Thursday.

Officials are also investigating the Cyberwarfare Command, a secretive branch of the South Korean army, after it was revealed that four of its officers engaged in a similar online smear campaign against Park's opponents. Investigators raided the Cyberwarfare Command's Seoul headquarters in late October, though it's unclear whether its smear campaign was carried out in coordination with the NIS. According to reports in Korean media, Cyberwarfare Command agents are accused of posting some 23 million tweets in favor of Park.

The controversy sparked widespread protests and demonstrations earlier this year, as South Koreans demanded that the NIS refrain from meddling in domestic politics. The country has a fraught history with the NIS, which was previously known under the acronym KCIA. Previous military dictators, including Park's late father, used the agency to quell dissidence through torture and to manipulate domestic politics.

Authorities have already indicted several high-ranking intelligence officials, including former NIS director Won Sei-hoon, who is accused of overseeing the operation. Park says she knew nothing of the campaign and has vowed to prevent the NIS from intervening in political matters. The NIS, meanwhile, says the tweets were posted as part of a routine operation against North Korea, which regularly uses the internet to spread criticism of South Korean policies.