The South Korean sites included the presidential Blue House, the Defense Ministry, the National Assembly, Shinhan Bank, Korea Exchange Bank and top Internet portal Naver. They went down or had access problems since late Tuesday, Ahn said.

Kudwa had no comment on the South Korean attacks.

Two government officials acknowledged that the Treasury and Secret Service sites were brought down, and said the agencies were working with their Internet service provider to resolve the problem.

Ben Rushlo, director of Internet technologies at Keynote Systems, called it a "massive outage" and said problems with the Transportation Department site began Saturday and continued until Monday, while the FTC site was down Sunday and Monday.

Keynote Systems is a mobile and Web site monitoring company based in San Mateo, Calif. The company publishes data detailing outages on Web sites, including 40 government sites it watches.

According to Rushlo, the Transportation Web site was "100 percent down" for two days, so that no Internet users could get through to it. The FTC site, meanwhile, started to come back online late Sunday, but even on Tuesday Internet users still were unable to get to the site 70 percent of the time.

"This is very strange. You don't see this," he said. "Having something 100 percent down for a 24-hour-plus period is a pretty significant event."

He added that, "The fact that it lasted for so long and that it was so significant in its ability to bring the site down says something about the site's ability to fend off (an attack) or about the severity of the attack."

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