Amid the growing suspicion that North Korea is behind Wednesday’s massive cyber attack, a government official says North Korea has adopted technologies that encrypt hackers’ communications and delete traces of attack.

The official said Thursday that the North takes a detour through network maintenance companies’ computers when attacking its cyber targets as South Korean national organizations now use closed-networks.

The government is believed to have identified North Korean cyber attack methods by analyzing North Korea’s previous DDoS attacks to South Korea in 2009 and in 2011, and its previous hackings into Nonghyup and the JoongAng Daily in 2011 and 12.

The official said that there is a possibility that the North could carry out simultaneous cyber attacks on South Korean transportation and electricity facilities.