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 US intelligence community, said that at least one of the federal agency Web sites got saturated with as many as 1 million hits per second per attack - amounting to 4 billion Internet hits at once. He would not identify the agency, but he 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.

The widespread attack was "loud and clumsy," which suggests it was carried out by an unsophisticated organization, said Amit Yoran, chief executive at NetWitness Corp. and the former US government cybersecurity chief. "This is not the elegance we would expect from sophisticated adversaries."

Officials agreed, however, that the incident brings to the forefront a key 21st century threat.

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