Investigators analyzed a USB memory stick seized from a key associate of the blogger, surnamed Kim and better known by his nickname Druking, and found that the team rigged comments on about 3,000 news stories at Daum and 100 articles at Nate, officials said.
Police have obtained court warrants to ensure that the operators of Daum and Nate preserve the related data amid concern that members of Druking's team could attempt to destroy evidence by removing the comments they manipulated.
Police plan to analyze data from the two portals to determine whether Druking's team used a "macro" computer program to rig the number of "likes" on comments on news stories carried by Daum and Nate, just as they did with stories on Naver.
The USB stick police seized from the Druking's associate, identified by the surname Kim, contained about 90,000 Internet addresses of stories targeted in manipulation attempts. Of them, about 71,000 were news stories carried by Naver and the rest included articles posted at Daum and Nate.
Druking has already been arrested for allegedly rigging the number of "likes" on one Naver story in January. Investigators have since pressed additional charges against him after finding that his team manipulated 20,000 comments on 676 news articles during the same period.
Further charges will be brought against him if the suspicions involving other portal site are confirmed.
The scandal gained greater attention following revelations that Rep. Kim Kyoung-soo of the ruling Democratic Party, who is considered one of the closet aides to President Moon Jae-in, knew and communicated with Druking.
Police questioned Kim earlier this month about his relationship with Druking, including whether he was aware of opinion rigging attempts and how he came to ask the presidential office to consider some candidates suggested by Druking as chief of an overseas diplomatic mission.
Kim has denied any wrongdoing.
Opposition parties have been demanding an independent counsel investigation into the scandal.(Yonhap)