In the past, cyberterrorists have shown a propensity to strike during international conflicts. Most have been minor invasions that are more annoying than disruptive, such as defacing Web sites or “denial of service” attacks that slow down or halt computer systems by flooding them with information.

But the potential exists for a more skilled cyberterrorist to disrupt systems for hospitals, power grids, banks and other key institutions.

Lawrence Gershwin, the CIA’s top adviser on science and technology issues, told Congress in June that traditional terrorists are less likely to pursue computer attacks. But that could change, he said.

The FBI’s National Infrastructure Protection Center is on the lookout for cyberterrorists and has issued a warning about “patriot hackers” — people trying to target those thought responsible for the terrorist attacks.

NIPC also is investigating the “Nimda” worm, a virus-like program that spread rapidly the week after the attacks and infected computers running Microsoft’s Windows operating system. Attorney General John Ashcroft has said there is no evidence linking the worm to the terrorist attacks.

Consultants like John Pescatore, research director for Internet security at Gartner Inc., have been advising clients on how to handle cyberterrorism threats, which he expects will occur if the U.S. military begins assaults on terrorist camps in Afghanistan and elsewhere.

He and other experts are advising companies to take steps now to avoid problems later. Among them: make sure they have the most recent anti-virus software, install security programs that include firewalls and encryption and increase user awareness.