WASHINGTON -- An unusually powerful electronic attack briefly crippled nine of the 13 computer servers that manage global Internet traffic this week, officials disclosed Tuesday. But most Internet users didn't notice because the attack only lasted one hour.

The FBI and White House were investigating. One official described the attack Monday as the most sophisticated and large-scale assault against these crucial computers in the history of the Internet. The origin of the attack was not known.

Seven of the 13 servers failed to respond to legitimate network traffic and two others failed intermittently during the attack, officials confirmed.

Service was restored after experts enacted defensive measures and the attack suddenly stopped.

The 13 computers are spread geographically across the globe as precaution against physical disasters and operated by U.S. government agencies, universities, corporations and private organizations.