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Experts said the unusually powerful attacks lasted as long as 12 hours but passed largely unnoticed by most computer users, a testament to the resiliency of the Internet. Behind the scenes, computer scientists worldwide raced to cope with enormous volumes of data that threatened to saturate some of the Internet's most vital pipelines.

The motive for the attacks was unclear, said Duane Wessels, a researcher at the Cooperative Association for Internet Data Analysis at the San Diego Supercomputing Center. "Maybe to show off or just be disruptive; it doesn't seem to be extortion or anything like that," Wessels said.

Other experts said the hackers appeared to disguise their origin, but vast amounts of rogue data in the attacks were traced to South Korea.

The attacks appeared to target UltraDNS, the company that operates servers managing traffic for Web sites ending in "org" and some other suffixes, experts said. Officials with NeuStar Inc., which owns UltraDNS, confirmed only that it had observed an unusual increase in traffic.

Among the targeted "root" servers that manage global Internet traffic were ones operated by the Defense Department and the Internet's primary oversight body.

"There was what appears to be some form of attack during the night hours here in California and into the morning," said John Crain, chief technical officer for the Internet Corporation for Assigned Names and Numbers. He said the attack was continuing and so was the hunt for its origin.

"I don't think anybody has the full picture," Crain said. "We're looking at the data."

Crain said Tuesday's attack was less serious than attacks against the same 13 "root" servers in October 2002 because technology innovations in recent years have increasingly distributed their workloads to other computers around the globe.