Panetta Warns of Dire Threat of Cyberattack on U.S. Defense Secretary Leon E. Panetta warned Thursday that the United States was facing the possibility of a "cyber-Pearl Harbor" and was increasingly vulnerable to foreign computer hackers who could dismantle the nation's power grid, transportation system, financial networks and government. In a speech at the Intrepid Sea, Air and Space Museum in New York, Mr. Panetta painted a dire picture of how such an attack on the United States might unfold. He said he was reacting to increasing aggressiveness and technological advances by the nation's adversaries, which officials identified as China, Russia, Iran and militant groups. "An aggressor nation or extremist group could use these kinds of cyber tools to gain control of critical switches," Mr. Panetta said. They could derail passenger trains, or even more dangerous, derail passenger trains loaded with lethal chemicals. They could contaminate the water supply in major cities, or shut down the power grid across large parts of the country. Defense officials insisted that Mr. Panetta's words were not hyperbole, and that he was responding to a recent wave of cyberattacks on large American financial institutions. He also cited an attack in August on the state oil company Saudi Aramco, which infected and made useless more than 30,000 computers. But Pentagon officials acknowledged that Mr. Panetta was also pushing for legislation on Capitol Hill. It would require new standards at critical private-sector infrastructure facilities - like power plants, water treatment facilities and gas pipelines - where a computer breach could cause significant casualties or economic damage. In August, a cybersecurity bill that had been one of the administration's national security priorities was blocked by a group of Republicans, led by Senator John McCain of Arizona, who took the side of the U.S. Chamber of Commerce and said it would be too burdensome for corporations. The most destructive possibilities, Mr. Panetta said, involve "cyber-actors launching several attacks on our critical infrastructure at one time, in combination with a physical attack." He described the collective result as a "cyber-Pearl Harbor that would cause physical destruction and the loss of life, an attack that would paralyze and shock the nation and create a profound new sense of vulnerability." Mr. Panetta also argued against the idea that new legislation would be costly for business. "The fact is that to fully provide the necessary protection in our democracy, cybersecurity must be passed by the Congress," he told his audience, Business Executives for National Security. Without it, we are and we will be vulnerable. With the legislation stalled, Mr. Panetta said President Obama was weighing the option of issuing an executive order that would promote information sharing on cybersecurity between government and private industry. But Mr. Panetta made clear that he saw it as a stopgap measure and that private companies, which are typically reluctant to share internal information with the government, would cooperate fully only if required to by law. "We're not interested in looking at e-mail, we're not interested in looking at information in computers, I'm not interested in violating rights or liberties of people," Mr. Panetta told editors and reporters at The New York Times earlier on Thursday. But if there is a code, if there's a worm that's being inserted, we need to know when that's happening. He said that with an executive order making cooperation by the private sector only voluntary, "I'm not sure they're going to volunteer if they don't feel that they're protected legally in terms of sharing information." "So our hope is that ultimately we can get Congress to adopt that kind of legislation," he added. Mr. Panetta's comments, his most extensive to date on cyberwarfare, also sought to increase the level of public debate about the Defense Department's growing capacity not only to defend but also to carry out attacks over computer networks. Even so, he carefully avoided using the words "offense" or "offensive" in the context of American cyberwarfare, instead defining the Pentagon's capabilities as "action to defend the nation." Elisabeth Bumiller reported from New York, and Thom Shanker from Washington.