North Korea a Suspect in Cyber Attacks in U.S.

There is likely some collaboration between North Korea, Iran and others on cyber warfare technology, Brooke said, but added that the likeliest culprits in the attacks are small-scale computer hackers rather than hostile governments.

"The choice of targets suggests that whatever group is doing it is sympathetic to North Korea," said Gene Spafford, executive director of Purdue University's Center for Education and Research in Information Assurance and Security.

This could include a "for-hire criminal group paid for by North Korea or sympathizers who could be anywhere in the world, including in South Korea, China, or even the U.S," he said.

The outages were caused by so-called denial of service attacks in which floods of computers all try to connect to a single site at the same time, overwhelming the server that handles the traffic, the Korea Information Security Agency said.

In South Korea, 12 sites were initially attacked Tuesday, followed by attacks Wednesday on 10 others, including those of government offices, banks, vaccine firms and Web portals, agency official Shin Hwa-su said.

The targets were all sites that could be accessed by the public, including the presidential Blue House, the Defense Ministry and some banks.

The U.S. targets included the White House, Pentagon, State Department, Treasury Department, Homeland Security and National Security Agency, as well as the New York Stock Exchange, Nasdaq stock market and The Washington Post.

Kim Yong-hyun, a professor at Seoul's Dongguk University and an expert on the North, said Pyongyang is believed to have advanced computer technology because the regime has put a key focus on information technology as a way to overcome its economic difficulties.

The country's absolute leader, Kim Jong Il, has been a force behind the push, saying those who don't use computers are among the "three main fools of the 21st century," along with smokers and anyone who doesn't appreciate music.

"If North Korea is found to be behind these attacks, it could mean that it tried to show the U.S. and the South that it has not only military capabilities, but also cyber capabilities to paralyze key facilities," said Kim, the professor in Seoul.

South Korea's main opposition Democratic Party accused the spy agency of leaking unconfirmed information in an attempt to build public support for a set of anti-terrorism bills that have been pending for months in the National Assembly amid opposition objections.

The opposition party claims the anti-terror bills would give the spy agency too much power and could be used as a tool to infringe upon human rights.

Peter Sommer, an expert on cyber-terrorism at the London School of Economics, cautioned against coming to quick conclusions as any instigator would disguise where the attacks were coming from.

"Initial diagnoses are often wrong," he said.

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Associated Press writers Jae-soon Chang and Wanjin Park in Seoul, Lolita C. Baldor in Washington, Pan Pylas, Gregory Katz, Nardine Saad and David Stringer in London and Karl Ritter in Stockholm contributed to this report.