The fact that a series of computers were involved in an attack, Reitinger said, "doesn't say anything about the ultimate source of the attack."

"What it says is that those computers were as much a target of the attack as the eventual Web sites that are targets," said Reitinger, who heads DHS cybersecurity operations. "They're just zombies that are being used by some unseen third party to launch attacks against government and nongovernment Web sites."

Targets of the most widespread cyber offensive of recent years also included the National Security Agency, Homeland Security Department and State Department, the Nasdaq stock market and The Washington Post, according to an early analysis of the software used in the attacks.

The Associated Press obtained the target list from security experts analyzing the attacks. They provided the list on condition of anonymity because they were not authorized to discuss the investigation.

Other experts in cyber assaults said the incident shined a harsh light on the US government's efforts to protect all of its agencies against Web-based attacks.

James Lewis, a senior fellow at the Center for Strategic and International Studies, said that the fact that both the White House and Defense Department were attacked but didn't go down points to the need for coordinated government network defenses.

"It says that they were ready and the other guys weren't ready," he said. "We are disorganized. In the event of an attack, some places aren't going to be able to defend themselves."

The wave of cyber assaults are known as "denial of service" attacks. Such attacks against Web sites are not uncommon and are caused when sites are so deluged with Internet traffic that they are effectively taken off-line. Mounting such an attack can be relatively easy and inexpensive, using widely available hacking programs, and they become far more serious if hackers infect and tie thousands of computers together into "botnets."

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