The outage came at the same time that, across the country, residents of East Coast cities were on high alert after U.S. intelligence officials confirmed Thursday they were investigating "credible" reports that at least three individuals entered the U.S. in August by air with the intent to launch a vehicle-borne attack against Washington, D.C. or New York around the anniversary of 9/11.
As a result, authorities in Arizona and California took pains to reassure customers there, quickly ruling out any suggestion of terrorism with the blackout.
"This was not a deliberate act. The employee was just switching out a piece of equipment that was problematic," said Daniel Froetscher, an APS vice president.
"We've never have this happen before, and we see no reason it will happen again," Dave Geier, vice president for San Diego Gas & Electric, said Friday.
"This was the most wide spread power outage ever in our service territory," he said. "The restoration process was a very difficult process. We had to build our transmission back piece by piece."
The power outage left hardest hit San Diego, the country's eighth largest city, in the dark during rush hour, jamming roadways and leaving commuters stranded as public methods of transportation like the city's famed trolley cars were shut down.
The area's largest airport, Lindbergh Field, closed for the night as businesses in the hardest-hit San Diego area shuttered their windows and hospitals and police stations brought in generators and struggled to keep operations going.
The blackout caused frustration and long lines at gas stations and the few business that did manage to stay open, but few arrests or cases of widespread looting were reported. All of the county's public schools remained closed Friday but will reopen on Monday.
Though Thursday's outage was the most extensive in California history, the outage is still dwarfed by a 2003 blackout in the Northeast that left more than 50 million people in the dark from Ohio to Canada and New York City. That blackout, also caused by a cascading series of power-line and electricity-plant failures, remains on record as the biggest blackout in U.S. history.
California's last experience with widespread blackouts came in 2001 when the state's failed experiment with energy deregulation was blamed for cutting power to more than 3 million customers for more than six days.
Thursday's outage brought attention to potential weaknesses with the electricity grid that services the San Diego County area in particular.
The power grid is part of a web of powerlines connecting electricity with their customers, from Canada down to northern Baja California. that extends from Canada down to northern Baja California, connecting electricity plants with their customers. San Diego is connected to the grid only through two major energy lines: a northern line connected to the San Onofre nuclear plants and an eastern line connected to power plants in Imperial County, Arizona, and northern Mexico.
ABC News' Lauren Effron contributed to this report.
The Associated Press contributed to this report.