Police are expanding their investigation into the cyber attack on the national election committee during a pivotal poll for Seoul mayor, as suspicions persist that the four arrested were acting on someone else’s behalf.

They were tracing bank accounts and phone records of the former assistant of Grand National Party Rep. Choi Ku-shik, identified by the surname Gong. Gong was among the four arrested on Saturday.

According to the National Police Agency’s cyber investigation unit, which is leading the probe, Gong is suspected of asking the chief of an IT company, identified by the surname Kang, to attack the National Elections Commission’s website.

Kang and two of his employees mounted a distributed denial of service attack, or DDoS, paralyzing the election body’s homepage for about two hours early Oct. 26. 

The 27-year-old Gong denies his involvement in the attack, while the other three have admitted guilt, investigators said.

Police found that Gong and Kang spoke on the phone nearly 30 times on the eve and early morning of the polling day. They are also trying to confirm whether the suspects are behind a similar attack on the same day against the home page of Park Won-soon. Park was elected as Seoul’s mayor.

The main opposition Democratic Party raised suspicions that it may have been a deliberate attempt to influence the outcome of the election.

In the Seoul mayoral by-election, Park, a liberal opposition-backed maverick, beat Na Kyung-won of the conservative ruling Grand National Party by a larger than expected margin, dealing a blow to President Lee Myung-bak and his GNP.

Many younger voters who wanted to cast their ballots on their way to work had trouble finding their polling booth, using smartphones or other wireless devices, because of the cyber attack, DP officials said. Surveys showed that voters in 30s and 40s gave an overwhelming support to Park.

The DP called the case “one of the worst election rigging attempts” and launched a task force Saturday, comprising of its lawmakers, vowing to get to the bottom of the attack.

Choi denied any involvement in the attack and said he would resign if the allegations were proven true. The GNP also said it has nothing to do with the attack.

By Lee Sun-young (milaya@herald.com)