Many analysts in Seoul suspect that North Korean hackers honed their skills in China and were operating there. At a hacking conference here last year, Michael Sutton, the head of threat research at Zscaler, a security company, said a handful of hackers from China “were clearly very skilled, knowledgeable and were in touch with their counterparts and familiar with the scene in North Korea.”

But there has never been any evidence to back up some analysts’ speculation that they were collaborating with their Chinese counterparts. “I’ve never seen any real evidence that points to any exchanges between China and North Korea, ” said Adam Segal, a senior fellow who specializes in China and cyberconflict at the Council on Foreign Relations,

Wednesday’s attacks, which occurred as American and South Korean military forces were conducting major exercises, were not as sophisticated as some from China that have struck United States computers, and certainly less sophisticated than the American and Israeli cyberattack on Iran’s nuclear facilities. But it was far more complex than a “denial of service” attack that simply overwhelms a computer system with a flood of data.

The malware is called “DarkSeoul” in the computer world and was first identified about a year ago. It is intended to evade some of South Korea’s most popular antivirus products and to render computers unusable. In Wednesday’s strikes, the attackers made no effort to disguise the malware, leading some to question whether it came from a state sponsor — which tend to be more stealthy — or whether officials or hackers in North Korea were sending a specific, clear message: that they can reach into Seoul’s economic heart without blowing up South Korean warships or shelling South Korean islands.

North Korea was accused of using both those techniques in attacks over the past three years.

The cyberattacks Wednesday come just days after North Korea blamed South Korea and the United States for attacks on some of its Web sites. The North’s official Korean Central News Agency said last week that North Korea “will never remain a passive onlooker to the enemies’ cyberattacks that have reached a very grave phase as part of their moves to stifle it.”