Computer networks at major South Korean banks and top TV broadcasters crashed en-masse Wednesday, paralyzing bank machines across the country and prompting speculation of a cyber-attack by North Korea.

Screens went blank at 2 p.m. (South Korean time), with reports of skulls popping up on some computer screens, the state-run Korea Information Security Agency said – a strong indication that hackers planted a malicious code in South Korean systems. Some computers came back online more than 2 1/2 hours later.

Police and South Korean officials couldn’t immediately determine the cause. But experts said a cyber-attack orchestrated by Pyongyang was likely to blame. The rivals have exchanged threats following U.N. sanctions meant to punish North Korea over its nuclear test last month.

cyber-attack
Employees of Korea Internet Security Center work after computer networks at two major South Korean banks and three top TV broadcasters went into shutdown mode en masse, at a monitoring room in Seoul, South Korea, Wednesday, March 20, photo by AP Photo/Yonhap, Han Jong-chan

The shutdown appeared to be more of an inconvenience than a source of panic. There were no immediate reports that bank customers’ records were compromised. It also didn’t affect government agencies or networks essential to the country’s infrastructure, such as power plants or transportation systems.