State prosecutors said Friday they have questioned three agents of the state intelligence agency on suspicion of posting political comments on Twitter ahead of the last presidential election.

The questioning is part of the prosecution's ongoing probe into a high-profile election meddling scandal allegedly involving the National Intelligence Service (NIS), they said.

The spy agency has been probed over allegation that it launched an online smear campaign to sway public opinion in favor of President Park Geun-hye, the then ruling party candidate, and slander the main opposition Democratic Party's candidate, Moon Jae-in, ahead of the December election.

The Seoul Central District Prosecutors' Office said it has arrested the three NIS agents for further questioning for their alleged involvement in tweeting and re-tweeting political articles on their Twitter accounts.

The three spy agency agents, however, were released after the NIS raised objection to the prosecution office that it was not properly notified of the arrests, sources at the prosecution office said.

Releasing a probe outcome, state prosecutors said in June that they have discovered a total of 320 tweets that allegedly praised Park and slandered Moon. The prosecution office has so far been investigating whether the accounts actually belonged to the NIS officials.

The prosecution office said it will focus its investigation to determine whether the three were part of a much bigger operation by the leadership of the agency. (Yonhap News)