Young said the attack seemed to be designed to interfere with its "DNS servers" that convert numerical Internet addresses into more recognizable names like "www.microsoft.com."

"Essentially an attacker would need to have enough [compromised computers] under his control to knock out the thousands of Akamai servers and achieve the kind of global outage we saw today," Johannes Ullrich, chief technology officer for the Bethesda, Md.-based SANS Institute.

Ullrich said the assault was most likely launched from an army of home computers infected with a virus or worm that gives attackers full control over the machines, which can then be used to send out spam or crippling Internet attacks like today's assault on Akamai.

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, Va., 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.

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 that 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 roadmap for almost all Internet communications. The Department of Homeland Security is still trying to find out who launched that attack, Yoran said.