"They've taken a theoretical attack and they've shown in a very demonstrable way the impact you can have using cyber means and cyber techniques against this type of infrastructure," said Amit Yoran, former U.S. cybersecurity chief for the Bush administration. Yoran is chief executive for NetWitness Corp., which sells sophisticated network monitoring software.

"It's so graphic," Yoran said. "Talking about bits and bytes doesn't have the same impact as seeing something catch fire."

The recorded demonstration, called the "Aurora Generator Test," was conducted in March by government researchers investigating a dangerous vulnerability in computers at U.S. utility companies known as supervisory control and data acquisition systems. The programming flaw was quietly fixed, and equipment-makers urged utilities to take protective measures.

There was no evidence any U.S. utility company suffered damage from hackers or terrorists using this technique, U.S. officials said. But these officials cautioned that affected systems are not routinely monitored as closely as many modern corporate computer networks, so there would be little forensic evidence to study after such a break-in.

"The video is not a realistic representation of how the power system would operate," said Stan Johnson, a manager at the North American Electric Reliability Corp., the Princeton, N.J.-based organization charged with overseeing the power grid.

A top Homeland Security Department official, Robert Jamison, said companies are working to limit such attacks.

"Is this something we should be concerned about? Yes," said Jamison, who oversees the department's cybersecurity division. "But we've taken a lot of risk off the table."

President Bush's top telecommunications advisers concluded years ago that an organization such as a foreign intelligence service or a well-funded terror group "could conduct a structured attack on the electric power grid electronically, with a high degree of anonymity, and without having to set foot in the target nation."