Security experts who studied the attacks said that it was part of the same campaign that took down the Web sites of JPMorgan Chase, Wells Fargo, Bank of America and others over the last six months. A group that calls itself the Izz ad-Din al-Qassam Cyber Fighters has claimed responsibility for those attacks.

The group says it is retaliating for an anti-Islamic video posted on YouTube last fall. But American intelligence officials and industry investigators say they believe the group is a convenient cover for Iran. Just how tight the connection is — or whether the group is acting on direct orders from the Iranian government — is unclear. Government officials and bank executives have failed to produce a smoking gun.

North Korea is considered the most likely source of the attacks on South Korea, though investigators are struggling to follow the digital trail, a process that could take months. The North Korean government of Kim Jong-un has openly declared that it is seeking online targets in its neighbor to the south to exact economic damage…

The American and South Korean attacks underscore a growing fear that the two countries most worrisome to banks, oil producers and governments may be Iran and North Korea, not because of their skill but because of their brazenness. Neither country is considered a superstar in this area. The appeal of digital weapons is similar to that of nuclear capability: it is a way for an outgunned, outfinanced nation to even the playing field. “These countries are pursuing cyberweapons the same way they are pursuing nuclear weapons,” said James A. Lewis, a computer security expert at the Center for Strategic and International Studies in Washington. “It’s primitive; it’s not top of the line, but it’s good enough and they are committed to getting it.”