Seoul Central District Prosecutors’ Office has ended an investigation into a hacking case that compromised about 35 million users, after no suspects were identified, Korea JoongAng Daily reports.

Hackers attacked major Korean portal site Nate and social networking site Cyworld, both managed by SK Communication, late in July 2011. According to Korea JoongAng Daily, the personal information of users – including IDs, names, cellphone numbers, e-mail adresses, encrypted social security numbers and encrypted passwords –  were leaked. Police traced the IP address to China but failed to identify a suspect.

Other websites in South Korea were targeted by unidentified hackers with 87.7% of the attacks coming from users with overseas IP addresses. Of those, 59.3% used Chinese IP adresses, Korea Internet Security Agency reported. The official website of the National Election Commission experienced a distributed denial-of-service (DDoS) ahead of general elections in April. State-run education broadcasting network was hacked in May. Both cases were traced back to China. South Korea said it is working with the Chinese government on the possibility of  restarting investigations.