The outage appears to have been caused by human error. But that fact is unlikely to comfort the growing number of people concerned about blackouts that could be triggered by cyber attacks.

Just this week for instance, the Bipartisan Policy Commission, a Washington-based think tank, expressed concern in a report about . The report said that attacks capable of triggering "cascading disruptions and damage" to U.S. power infrastructure are not just theoretical threats, but a very real danger.

The southern California outage started around 5.30 p.m ET Thursday and left close to 1.5 million customers of San Diego Gas & Electric (SDG&E) without power for nearly 12 hours. As of this afternoon, power had been restored to all affected areas.

In a news conference streamed live by local TV stations this afternoon, SDG&E president Mike Niggli said the problem started when an employee with APS, Arizona's largest utility company, was working on a capacitor, at a substation near Yuma.

"It was a human error that was the initiating event that took the transmission line out," Niggli said, nothing that the incident caused power flows to be redistributed throughout large portions of the western United States. "We know what the initiating event was. The question is, how did that ripple through the rest of the systems?"