It's too early to say for sure what happened with yesterday's blackout, said Joseph Weiss, managing partner at Applied Control Solutions LLC and author of the book Protecting Industrial Control Systems from Electronic Threats.

But the key takeaway is that a cascading blackout can just as easily be triggered by a malicious act as by human error, he said. "The only way you can tell the difference is the intent of the individual," he said.

Often cyber analysts tend to view threats to the power grid in the same way they view threats to information networks, he said. Any incident that results in an industrial control system being taken offline because of something happening upstream is, in a sense, a cyber incident.

In his book, Weiss says that there have been at least 170 known cyber-related outages in the U.S., including three that caused widespread regional outages. The relative lack of forensics-gathering capabilities in the utility business makes it hard to determin whether any of them might have been the result of a malicious act, he said.

"Because we have so little control systems forensics, it is very difficult to determine what happened" with many of these incidents, Weiss said.