SEOUL, South Korea — At least two agents from the South Korean National Intelligence Service illegally posted comments online criticizing the political opposition ahead of the December presidential election, the police said on Thursday in an interim report on an investigation into accusations of political meddling.

The police said it remained unclear whether the two agents were part of an operation to influence the Dec. 19 election, as the opposition Democratic United Party claimed. But the findings were a blow to President Park Geun-hye, who had vehemently accused her opposition rival, Moon Jae-in, of a political offensive when his party first made accusations of illegal campaign activities by intelligence agents.

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.