Evildoers commandeer thousands of home computers, creating a virtual army that knocks down chunks of the Internet. Computer infections hit a nuclear plant, crash a 911 system, snarl train service and shut down ATMs. A neighborhood glitch compromises air traffic control computers.
It's all happened before, security experts say.
Luckily for America, it hasn't happened all at once — yet.
There is skepticism, but some fear it could. The recent accidental power outage which took out tens of millions of electricity consumers also spurred concerns.
"The Northeast power blackout … could happen as a result of a terrorist attack using cyber [methods]," said Richard Clarke, America's former cybersecurity czar, now an ABCNEWS consultant.
"[There are] a lot of people in the Department of Homeland Security that believe the only terrorist events worth worrying about are the ones with explosions and bodybags, and that's a very 20th-century way of looking at the problem," Clarke added. "In the 21st century, cyberspace is what controls the country."
Many key targets may be accessible online, said Alan Paller, director of research for the SANS Institute, which trains computer security experts.
"Because of the need for remote management, a very large percentage of the systems that run the critical infrastructure are connected to the Internet, in spite of claims to the contrary by officers of those companies; they just don't know," Paller said.
The federal government is concerned — spending just under $5 billion per year to protect its cyber networks from teenage hackers, identity thieves, foreign enemies and terrorists alike, Clarke said. Private industry also is spending huge sums to defend against attacks that could conceivably cripple the stock market, air travel or power plants.
Terrorist groups have never launched a documented cyberattack against American interests, but al Qaeda appears to have done online reconnaissance in search of U.S. vulnerabilities, Clarke said. In addition, hacking tools have been found on seized al Qaeda computers, and the terror group is said to use the Internet as a communications tool.
But some computer security professionals note the lack of prior al Qaeda cyber attacks, and have a hard time imagining a terrorists using cyber methods for a 9/11 style attack.
"If their goal was to disrupt things, there are easier, cheaper ways … that don't leave footprints all over the Internet" to tip off authorities, said Jeff Moss, organizer of the annual Def Con computer hacker's convention, and owner of Black Hat, a computer security company.
On the other hand, it may not take al Qaeda to terrorize.
"The effects of a software programming error, the effects of somebody making a mistake or the effects of someone attacking, they're all the same," said Winn Schwartau, author of Information Warfare: Chaos on the Electronic Superhighway, and numerous other books on the subject.
"If, for example, somebody modifies a whole lot of records at a hospital … and suddenly a lot of prescriptions and information about the patients are changed, and they start dying off, do you call that terror?" Schwartau asked. "If you tamper with a nuclear reactor, is that terror?"
Foreign governments also may be a concern. Chinese officials have said they consider cyber attacks to be a potential war weapon, and they could be a part of the U.S. arsenal as well, Clarke suggested.