The prosecution on Tuesday banned officials who are accused of illegally surveilling a civilian from leaving the country as it launched a full investigation into the case.
Lee In-kyu, a former senior official at the prime minister's office in charge of monitoring ethics code violations by public officials, and three others were referred to the prosecution Monday on suspicion of conducting a probe into the civilian who posted a video critical of President Lee Myung-bak on his Internet blog in 2008.
The video, made by a Korean student in the United States, was popular on YouTube, recording more than 2 million hits at the time.
The prime minister's office relieved three of them, including Lee, from their posts for failing to confirm beforehand that the blogger, Kim Jong-ik, was a civilian, not a government official.
Prosecutors said they will soon summon Kim and the accused officials to look into the allegations.
"We will proceed with the probe as quickly as possible to get to the bottom of the suspicions," a prosecution official said.
Chung Sye-kyun, chairman of the main opposition Democratic Party (DP), on Tuesday renewed his demand for a parliamentary investigation and appointment of a special counsel to conduct a probe.
"No organization of the incumbent regime is qualified to look into the case. The National Assembly must investigate it," Chung said in a radio address.