An apparently coordinated attack on the computers that act as the backbone of the Internet briefly crippled some of them Monday evening, but despite the scale of the attack, Internet users were largely unaffected.

The FBI said it was investigating the "distributed denial of service" attack. David Wray, a spokesman for the FBI's National Infrastructure Protection Center, would not speculate on who might have carried out the attack.

Such attacks overwhelm networks with data until the networks fail. Seven of the 13 servers failed Monday, and two others failed intermittently during the hour-long attack, which began about 5 p.m., said officials who operate some of the root servers. The root servers, located around the world, serve as a master directory for the Internet, converting numeric codes into the words and names that form e-mail and Web addresses.

Safeguards built into the Internet's architecture require eight or more root servers to fail before ordinary users observe slowdowns.

"It's not something that you see every day, but it's not something that couldn't be handled," said Louis Touton, vice president for the Internet Corporation for Assigned Names and Numbers, the Internet's key governing body. "There was no service interruption from anyone we've talked to."

The attacks are some of the most common and easiest to perpetrate, but the size and scope of Monday's strike set it apart from others, with some sources saying it was the largest such attack on the Internet root servers.

Internet Software Consortium Inc. Chairman Paul Vixie, who called this attack "the largest in recent memory," noted the rarity of an attack against all the root servers at once.

In February 2000, such attacks halted, eBay, Yahoo and other big-name e-commerce sites for several hours. In August 2000, four of the 13 root servers failed briefly because of a technical glitch. A more serious problem occurred in July 1997 after experts transferred a garbled directory list to seven root servers and failed to correct the problem for four hours. Traffic on much of the Internet ground to a halt.

Washington Post staff writer Jonathan Krim and Robert MacMillan of contributed to this report.