Online attack hits US government Web sites

was averaging about 1.2GB per second, he said.

Security experts estimate the size of the botnet at somewhere between 30,000 and 60,000 computers.

It is also unusual to see relatively low-profile government Web sites being hit. "Who goes around targeting a site like the FAA or the U.S. Treasury? It's not something that most people would think to attack," Stewart said.

The FTC in the past has brought actions against spammers and Internet fraudsters. Last month it shut down an Internet service provider called Pricewert, which had been associated with botnets, spam and child pornography.

No one knows who is behind the attack, although Stewart said it could have been launched by a single person. "It just seems to me that somebody is mad for some reason at capitalist governments," he said. Security experts say most of the infected machines are located in South Korea, but that doesn't mean the attack originated there.

The fact that the DDoS attack took down government computers is an embarrassment to the U.S., which is working to strengthen the country's cyber-security defenses under President Barack Obama.

"These are very basic attacks and stuff we've seen for a very long time. The scale of these isn't very huge either," said one security expert, who spoke on condition of anonymity because he wasn't authorized to discuss the matter publicly. "It's embarrassing that these sites have been hit for four or five days and they're still being affected. Think of the money that eBay and Amazon would lose in four to five days of this."

(Grant Gross in Washington and Nancy Gohring in Seattle contributed to this story.)