A widespread electronic attack on a company that handles traffic for some of the world's most-visited Web sites knocked several prominent sites offline for at least 45 minutes early yesterday.

The attack targeted Internet servers run by Cambridge, Mass.-based Akamai Technologies Inc., which distributes and manages Web data for companies such as Microsoft Corp., Yahoo Inc., Federal Express Corp. and Xerox Corp. It also handles traffic for the FBI and washingtonpost.com.

Akamai spokesman Jeff Young said the attack interrupted service to the Web sites around 9 a.m. and lasted for just under an hour.

Young said the attack was targeted at Internet networks on a broad scale, adding that "we have no reason to believe that the attack was directed solely at Akamai."

Amit Yoran, chief cybersecurity officer for the U.S. Department of Homeland Security, said federal authorities are working with Akamai and the companies that operate the Internet's underlying infrastructure to determine the source of the attack.

Akamai manages high-traffic Web sites by storing its 1,100 customers' Web content on thousands of Internet servers around the world. It manages approximately 15 percent of the traffic on the Internet.

Young said that most of the sites that were affected are search engines that use Akamai's services.

The company's role makes it an attractive target for hackers who attempt to overwhelm computers and Web sites by flooding them with huge bursts of data. Often, such attacks originate from computers that have been infected with a worm or virus designed to launch an assault at a set time.

Security experts have been warning about the growing number of computers infected with such programs. One of the most aggressive and powerful such programs, called Phatbot, has already spread to millions of machines over the past several months.

Russ Cooper, chief scientist at TruSecure Corp. in Herndon, said the attack probably involved "at least tens of thousands of systems that would be needed to busy Akamai's network so much."

Cooper said the attackers also might have targeted a previously unknown design flaw in Akamai's software.

Young said the attack seemed to have been designed to interfere with the company's domain-name system (DNS) servers, machines that convert numerical Internet addresses into more recognizable names such as "www.microsoft.com."

The company said that a similar incident last month was caused by a software flaw in one of its Web-site management programs.

Computer security experts and law enforcement authorities said it is often extremely difficult to find out who is responsible for denial-of-service attacks.

In October 2002, a denial-of-service attack disabled most of the 13 root servers that provide the primary road map for almost all Internet communications. The Department of Homeland Security is still trying to find out who launched that attack, Yoran said.

Krebs is a staff writer for washingtonpost.com.