The cyber attack did not appear, at least at the outset, to target internal or classified files or systems, but instead aimed at agencies' public sites, creating a nuisance both for officials and the Web consumers who use them.

Ben Rushlo, director of Internet technologies at Keynote Systems, 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. 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.

Dale Meyerrose, former chief information officer for the U.S. intelligence community, said that at least one of the federal agency Web sites got saturated with as many as a million hits per second per attack — amounting to 4 billion Internet hits at once. He would not identify the agency, but said the Web site is generally capable of handling a level of about 25,000 users.

Meyerrose, who is now vice president at Harris Corp., said the characteristics of the attack suggest the involvement of between 30,000 to 60,000 computers.

He said it appears there was one attack on July 4, which some agencies were able to contain, and then a second round on Tuesday. Meyerrose said that since the attackers would have used surrogate computers, it is still too early to tell where it originated.

South Korea's National Intelligence Service, the nation's principal spy agency, told a group of South Korean lawmakers Wednesday it believes that North Korea or North Korean sympathizers in the South were behind the attacks, according to an aide to one of the lawmakers briefed on the information.

The aide spoke on condition of anonymity, citing the sensitivity of the information. The National Intelligence Service — South Korea's main spy agency — said it couldn't immediately confirm the report.

Amy Kudwa, spokeswoman for the Homeland Security Department, said the agency's U.S. Computer Emergency Readiness Team issued a notice to federal departments and other partner organizations about the problems and "advised them of steps to take to help mitigate against such attacks."

New York Stock Exchange spokesman Ray Pellecchia could not confirm the attack on the trading institution, saying the company does not comment on security issues.