Sept. 9, 2011 -- Residents of Arizona, Southern California and Mexico woke up with their lights back on this morning after a major power outage sparked by a single utility company worker left millions of them in the dark overnight.
Officials with San Diego Gas & Electric Co. confirmed that, as of around 4:30 a.m. PST, all power had been restored to the area's 1.4 million customers. Power was restored late Thursday to millions of consumers in Arizona and parts of California's Orange and Imperial counties also affected by what officials described as a "cascading" blackout, and the worst in California history.
The massive blackout, which shut schools, grounded planes and brought business and traffic to a halt, was caused by one utility worker performing routine maintenance at a power substation near Yuma, Ariz.
The result, from the deserts of Arizona, was a 12-hour, full system blackout for 5 million people across San Diego County, southern Orange County, western Arizona, northern Baja California and even into areas of Mexico.
The problem began just before 3:30 p.m. Thursday when a worker for Arizona Public Service, the state's largest utility provider, replaced monitoring equipment that had been causing problems, creating a short-term power outage for about 56,000 customers in Yuma and western Arizona.
Ten minutes later, an attempt by workers at the same substation to restore power was unsuccessful and, instead, shortened the circuits.
Because the substation is a main point of delivery between power plants in Arizona and San Diego, the electric system shut down across the Southwest in what officials with San Diego Gas & Electric described as a "cascading effect."
The blackout extended south of the border to Tijuana, Mexicali and other cities in Mexico's Baja California state, which are connected to the U.S. power grid and have a combined population of 2.5 million people who were affected.
Typically, in such an instance, the outage would be isolated to the Yuma area. The investigation is now focusing on the reason that did not occur in this case, APS said Thursday.
The loss of power led to a shutdown of two reactors at the San Onofre nuclear power plant. Officials from the U.S. Nuclear Regulatory Agency said it appeared to have shut down automatically at 3:38 p.m. because of the change in the power grid -- as it is designed to do, kind of like a circuit breaker. Officials worked overnight to reconnect the reactor so that it could help restore power to some of the many people affected.
There are safeguards built into electrical systems across the country," said Stephanie Donovan, a spokeswoman for SDG&E. "When the power plant shut off at San Onofre, it was a built-in, automatic protective device. ... The reason that the outage tends to cascade is that the system tried to protect itself from voltage fluctuations."
Officials Deny Terrorism Link
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.