Ms. Park, the governing party candidate, won the election by a margin of one million votes.

The case revived long-held suspicions among South Koreans over the role of the National Intelligence Service. The country’s former military dictators — including Ms. Park’s father, the late President Park Chung-hee — had used the agency, once known by its infamous acronym, K.C.I.A., to torture and silence dissidents and influence domestic politics.

After the country democratized in the early 1990s, the agency, which has changed its name a few times, repeatedly vowed not to intervene in politics.

On Thursday, Lee Kwang-seok, chief of the Suseo Police Station in Seoul, admitted difficulties investigating the secretive agency. The supervisor of the two agents, who are from the intelligence service’s psychological intelligence bureau, refused to be questioned, Mr. Lee said.

The police asked prosecutors to formally indict the two agents, whose names were not released, on charges of violating a law that requires intelligence officers to maintain political neutrality. A third person, not affiliated with the agency, faces a criminal charge of helping the agents in their online operation.

The police, citing a lack of evidence, stopped short of accusing the agents of a more politically volatile crime of violating the country’s election law, a decision the opposition party called a whitewash.

Political parties had earlier agreed to conduct a separate parliamentary inquiry. Prosecutors have also barred the former intelligence service director, Won Sei-hoon, a close ally of former President Lee Myung-bak, from leaving the country.

There was no immediate reaction from Ms. Park’s office or the intelligence service. The agency had earlier denied interfering in the election. It said its officers’ online activities had been part of its normal psychological operations aimed at North Korea.

Park Yong-jin, spokesman for the Democratic United Party, said Thursday that the case showed that the agency was “'a chambermaid of political power,” and compared the campaign activities of which it is accused to a “coup d'état.”

Mr. Park also accused the national police of dragging their feet in investigating the case, out of fear of offending President Park. The police said the investigation was continuing.

The case began when police officers and officials from the National Election Commission knocked on the door of a room in an office and residential complex in southern Seoul on Dec. 11, just over a week before the election. They were responding to a tip from the opposition party that a 29-year-old agent was running an illegal online election campaign operation from there.

But they could not even enter the room, as the agent had locked herself in. A political standoff erupted. The opposition accused the intelligence service of blocking an investigation. Ms. Park and her party accused the opposition of harassing the woman. The police took two days to obtain two computers from the woman and another two days before questioning her for the first time.

Three days before the election, the police said they had found no evidence of illegal online activities. After the election, however, the police said further investigations revealed that the woman used 16 Internet user IDs to upload numerous comments often criticizing opposition candidates on politically sensitive issues. Still later, they questioned a second agent and a person who was said to have been hired by the agents to assist them in their work.

http://www.nytimes.com/2013/04/19/world/asia/police-in-south-korea-say-spy-service-tried-to-influence-election.html?_r=0