Chinese hackers are suspected of attacking the White House website
President George W Bush appears to be the latest victim in an online assault by Chinese hackers, as the White House website was unavailable for several hours early Friday.
The hackers' attack was similar to ones that took CNN and Yahoo offline last year.
The White House confirmed that for two hours and 15 minutes their website was down.
A White House spokesman said "there was no security breach, and the attack is under review".
The attack began between 1100 and 1200 GMT, said Dan Todd of Keynote Systems, which monitors website performance.
The company has computers all over the United States that monitor the responsiveness of websites.
"Beginning about 0445 am Pacific Time, the (White House) server stopped responding," Mr Todd said.
White House targeted
Hackers in the US and China have been engaged in an online battle this week.
Chinese hackers have defaced more than 660 sites in the past week, according to Michael Cheek with security firm iDefense.
The hackers promised a cyber-offensive against US sites in observance of Chinese of Labour Day on 1 May and Youth Day on 4 May, and also in remembrance of the US bombing of the Chinese embassy in Belgrade two years ago on 7 May.
The Federal Bureau of Investigations had warned of the increased threat, and this week it said it had seen a significant increase in attempted attacks.
Sites are being actively scanned for weaknesses. "These probes and attempted exploitations currently number in the millions and the activity is ongoing," according to the FBI's National Protection Infrastructure Centre.
The firm iDefense has been monitoring the activity of Chinese hackers. It put out an alert on 30 April, saying the hackers would specifically target 14 high-profile government and commercial websites.
Government sites targeted included the White House, the CIA, the Pentagon, US Navy, and National Security Agency.
The commercial targets included The New York Times, CNN and The Army Times.
The alert said: "The White House is given special attention, with a co-ordinated attack planned for Friday, May 4."
More than online vandalism
Up until now US and Chinese hackers have only focused on defacing websites.
US hackers would leave anti-China messages and call on the Chinese to return the surveillance plane crippled in a collision with a Chinese fighter.
The Chinese hackers have been distributing a denial of service attack tool on their website. Such attacks direct a huge amount of traffic at a website, rendering it unable to process legitimate requests.
The method was used to cripple several high-profile sites last year, including CNN and Yahoo!
Chinese hackers had been trying to deface the White House website for much of the week, Mr Cheek said. They even expressed frustration at the difficulty they were having in either defacing or crippling it, he added.
"It was more protected than they thought," he added.
The Chinese hackers have been actively distributing what Mr Cheek described as a "flood" type of tool and were giving detailed instructions on how to configure the tool for the attack.
There is also some evidence that Chinese hackers are not merely defacing websites but also destroying data.
"In some cases, they leave behind script or something so when rebooted it wipes the server," Mr Cheek said.
US hackers have not sat idle during this period and have stepped their attacks over the past week as well, he added.
US hacker attacks began shortly after the collision of a US surveillance plane with a Chinese fighter jet last month. In the past 30 days, more than 450 Chinese sites have been defaced.