Prosecutors on Monday raided an office of the Seoul police in connection with a female detective's claims that high-ranking police officers hampered an investigation into the nation's intelligence agency's alleged attempt to influence public opinion ahead of last year's presidential election.

A team of prosecutors and investigators of the Seoul Central District Prosecutors' Office raided the cyber crime unit of the Seoul Metropolitan Police Agency (SMPA) and seized computer hard discs and relevant documents to verify the claims, officials 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.

Last month, 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 to treat the case lightly.

The superiors pressed her team to drastically decrease the number of search words for analyzing one of the NIS officers' computer hard discs, 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.

After analyzing seized documents, prosecutors will soon call in former SMPA Chief Kim Yong-pan to determine whether he was involved in the alleged cover-up attempt. (Yonhap News)