Computer networks at two major South Korean banks and three top TV broadcasters went into shutdown mode en masse Wednesday, paralyzing bank machines across the country.Screens went blank promptly at 2 p.m. (0500 GMT), with skulls popping up on the screens of some computers - a strong indication that hackers planted a malicious code in South Korean systems, the state-run Korea Information Security Agency said. Some computers started to get back online more than 2 ½ hours later.Police and South Korean officials investigating the shutdown said the cause was not immediately clear. But speculation centered on North Korea, with experts saying a cyber attack orchestrated by Pyongyang was likely to blame, according to a report of the Associated Press.