The hacker attempt on the Whitehouse.gov address, which crashed the site, lasted from 11 a.m. until after 5 p.m. PDT Tuesday. The White House confirmed that the barrage of page requests created through the attack was "heavy enough to block most legitimate users."
On May 4, the Whitehouse.gov site was down for more than three hours because of a similar attack, purported to be linked to a string of Web site hacks and defacements organized by Chinese and pro-Chinese Internet vandals during the first week of May.
The Hackers Union of China admitted responsibility for numerous attacks on U.S. government and commercial sites at the start of May. It has not been confirmed whether the same group was behind Tuesday's attack.
Staff writer Wendy McAuliffe reported from London.