SEOUL, South Korea -- Military investigators raided South Korea's Cyberwarfare Command on Tuesday after four of its officials were found to have posted political messages online last year, in what opposition lawmakers have called a smear campaign against President Park Geun-hye's opponents before her election in December.
Ms. Park defeated her main opposition rival, Moon Jae-in, by roughly 1 million votes in the election and took office in February. But in a snowballing scandal, prosecutors have since said that during the presidential campaign, National Intelligence Service agents posted thousands of Internet messages supporting Ms. Park and her governing Saenuri Party or berating government critics, including opposition presidential candidates, as shills for North Korea.
Last week, opposition lawmakers alleged in the National Assembly that the military's secretive Cyberwarfare Command had carried out a similar online campaign, separately or in coordination with the spy agency, to help sway public opinion in favor of Ms. Park before the Dec. 19 election.
On Tuesday, the Defense Ministry confirmed that four cyberwarfare officials had posted political messages. But it quoted them as saying they had acted on their own. Still, "the ministry will investigate whether there was command-level involvement," the ministry's spokesman, Kim Min-seok, said in explaining the raid on the command headquarters.
The cyberwarfare command, created in 2010 to guard South Korea against hacking threats from North Korea, is only the latest state agency to have been touched by the scandal.
The political opposition first raised the charge of illegal electioneering during the presidential campaign last year. Three days before the voting, however, police announced that they had investigated and found no evidence to support the accusations.
But in June, prosecutors indicted Won Sei-hoon, the intelligence agency's former director, for allegedly supervising an online smear campaign against Ms. Park's political foes. They also indicted Kim Yang-pan, former chief of the Seoul Metropolitan Police, saying he had whitewashed junior officers' investigation into the matter.
Mr. Won and the spy agency insisted that the online messages were posted as part of normal psychological warfare operations against North Korea and did not amount to meddling in an election. Ms. Park has denied using the spy agency for her campaign.
Since Ms. Park's inauguration, South Korean politics has have been paralyzed by scandals, including the one surrounding the spy agency. Rival political rallies have rocked downtown Seoul in recent weeks. Student activists demanded reform within the intelligence agency to prevent it from meddling in domestic politics.
But older, conservative Koreans have encouraged the agency, known by its acronym NIS, to "wipe out North Korea followers" from the National Assembly and cyberspace. Last month, the spy agency arrested a far-left nationalist opposition lawmaker on charges of plotting an armed rebellion against the South Korean government in the event of war with North Korea.
The scandal has kept growing. Last month, a Seoul court ordered the prosecution of two more senior intelligence officials for involvement in the alleged online campaign.
On Monday, during a National Assembly hearing, Yoon Seok-ryeol, a senior prosecutor who had led the scandal inquiry until recently, said his team had been under "external pressure."
Mr. Yoon was removed from the investigation last week, after his team detained three intelligence agents and searched their homes. He said his team had collected more evidence of the spy agency's online campaign: 55,700 messages, posted or reposted by intelligence agents, that praised Ms. Park or disparaged her opposition rivals ahead of the December election. One of them called Mr. Moon, the main opposition candidate, a "servant" of North Korea and cited Ahn Cheol-soo, an independent who supported Mr. Moon, as "a woman in men's clothes."
Cho Yong-gon, head of the Seoul District Prosecutor's Office, who supervised Mr. Yoon, denied putting pressure on his team. He said Mr. Yoon had been pulled from the inquiry because he had not discussed the spy agents' detentions in advance with superiors, as regulations require.