Prosecutors summoned a former head of the Seoul police agency Tuesday for questioning over suspicions that he had hampered a police probe into the nation's spy agency's alleged attempt to influence public opinion ahead of last year's presidential election.

The summons came after a female detective, who led the police investigation, claimed that high-ranking police officers of the Seoul Metropolitan Police Agency (SMPA) had pressured her to treat the case lightly.

Kim Yong-pan, who headed the SMPA at the time, appeared for questioning at the Seoul Central District Prosecutors' Office in southern Seoul around 10:00 a.m., prosecutors said.

Police have been probing 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.

On April 18, Seoul's Suseo Police Station, which investigated the case for four months, announced that at least two NIS agents illegally intervened 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.

Kwon Eun-hee, the lead investigator on the case at the time, claimed a day later that she could not fully look into the case due to immense pressure from her superiors.

On Monday, a team of 27 prosecutors and investigators raided the headquarters of SMPA in central Seoul, seizing computer hard drives and relevant documents to verify the claims.

The superiors pressed her team to drastically decrease the number of search words for analyzing one of the NIS officers'

computer hard drives, Kwon said.

Prosecutors have questioned Kwon and her bosses, including the former head of the Suseo Police Station, over her claims since early this month. (Yonhap News)