A detective who blew the whistle on the alleged police attempt to cover up a widening spy agency scandal appeared for questioning by prosecutors Wednesday.

The scandal centers around allegations that the National Intelligence Service (NIS) systemically and extensively meddled in the presidential election by using its agents to illegally post a slew of politically sensitive comments against the opposition candidate on the Internet to sway public opinion ahead of the December vote.

Wrapping up a four-month-long probe, Seoul's Suseo Police Station announced last month that only two NIS agents violated the law against the main spy agency intervening in domestic politics by posting political comments and replies on various Web sites ahead of the election. But it cleared them of charges of violating the election law under which they could face heavier punishment.

A day later, Kwon Eun-hee, the lead investigator in charge of the case at the time, claimed that she could not fully look into the case due to undue pressure from high-ranking officials to go easy on it.

Kwon entered the Seoul Central District Prosecutors' Office in southern Seoul around 2 p.m. Wednesday to be questioned on what kind of pressure there was and the extent of interference from the top echelons.

The exposure immediately stoked vehement protests from the main opposition Democratic Party, which condemned the authorities and demanded a thorough probe into former Seoul Metropolitan Police Agency (SMPA) Chief Kim Yong-pan and other high-ranking officials for their possible involvement.

As an example of undue influence from her superiors, Kwon said that her team initially had asked the SMPA to analyze all computer files that included 78 key words but the agency later cut down the number to four.

The SMPA officials have refuted Kwon's claims, arguing that they had never unilaterally asked her to reduce the number.

Kwon was transferred to the Songpa Police Station in February before the investigation ended, a move suggesting that there could have been behind-the-scenes influences on the investigation.

Won Se-hoon, former NIS chief, was questioned last week by the prosecution over the allegations but denied any wrongdoing by the agents, saying what they did was part of their duty of monitoring North Korea's psychological warfare. 

Won asked for a police investigation after firebombs were thrown at his house in northern Seoul early Sunday morning. Police said two fire-lit bottles were thrown into his garden but they flickered out without causing damage. It wasn't yet clear if the incident was related to the ongoing probe. (Yonhap News)