Clarke, in an e-mail sent overnight Thursday to colleagues, cited damage from the weekend's infection that struck hundreds of thousands of computers worldwide, slowing e-mail and Web surfing and even shutting down some banking systems. He called the attacking software "a dumb worm that was easily and cheaply made."
"More sophisticated attacks against known vulnerabilities in cyberspace could be devastating," Clarke wrote. "As long as we have vulnerabilities in cyberspace and as long as America has enemies, we are at risk of the two coming together to severely damage our great country."
A spokeswoman confirmed Clarke's e-mail as authentic. It was forwarded by the FBI's National Infrastructure Protection Center to operators of Internet early-warning centers.
Leading experts are skeptical that the FBI and other investigators will be able to track down whoever was responsible for last weekend's attack.
The virus-like disruption crippled 911 and financial institution systems, and even Microsoft itself.
"It is scary to think that it affected police and fire dispatches," said CBS Radio News Tech Analyst Larry Magid. "Nobody died as far as we know as a result of the attack, but certainly a lot of people were inconvenienced. It had a huge impact on South Korea's economy, and we don't yet know what impact it had on the U.S. economy."
Police and fire dispatchers outside Seattle resorted to paper and pencil for hours after the attack disrupted operations for the 911 center that serves two suburban police departments and at least 14 fire departments.
The attack prevented many customers of Bank of America Corp., one of the largest U.S. banks, and some large Canadian banks from withdrawing money from automatic teller machines Saturday. The nation's largest residential mortgage firm, Countrywide Financial Corp., Microsoft and American Express Co. also reported problems.
Experts, including many who provide technical advice to the FBI and other U.S. agencies, said exhaustive reviews of the blueprints for the attacking software are yielding few clues to its origin or the author's identity.
"The likelihood of being able to track down the specific source of this is very unlikely," said Ken Dunham, an analyst at iDefense Inc., an online security firm. "We don't have the smoking gun."
Many top experts believe the programming for the Internet worm was based on software code published on the Web months ago by a respected British computer researcher, David Litchfield, and later modified by a virus author known within the Chinese hacker community as "Lion."
The worm's author could face up to life in prison under new U.S. anti-terror legislation passed two months ago, some legal experts said.
The Associated Press, citing people familiar with Clarke's plans, reported his decision to resign on Jan. 24. Clarke has spent 11 years in the White House across three administrations, and he was the president's counter-terrorism coordinator at the time of the Sept. 11 terror attacks.
Clarke has focused most recently on preventing disruptions to important computer networks from Internet attacks, compiling recommendations to improve security into a "National Strategy to Secure Cyberspace." In his e-mail, Clarke urged companies and government agencies to adopt these recommendations.
He said it was "essential to the health of the nation's economy and the security of the country."
Clarke indicated he would seek a job in the private sector, after spending three decades inside the government. He worked at the Departments of Defense and State, then was hired at the White House.