More than 1.4 million people plunged into darkness during a power outage that encompassed the San Diego area and beyond can blame an electric utility employee in Arizona.

A power line failed when the employee was conducting regular maintenance at a substation near Yuma, Arizona, according to Arizona Power Service, the state's largest electric utility provider.

The outage appears to be related to a procedure an APS employee was carrying out in the North Gila substation, which is located northeast of Yuma, an APS press release said. Operating and protection protocols typically would have isolated the resulting outage to the Yuma area. The reason that did not occur in this case will be the focal point of the investigation into the event, which already is underway.

That transmission failure rippled out into sections of the electric grid as far west as San Diego and as far south as Mexico, with San Diego losing power after the San Onofre Nuclear Generating Station shut down. San Diego Gas & Electric Co. said on Friday that it had restored power to customers who had faced the most widespread power outage in the company's history but urged customers to restrict their electricity use.

After the company has ensured the power system is safe, it will turn its attention toward determining the sequence of events that led to the outage and establishing practices and procedures to ensure that outages such as the Sept. 8 event are not repeated, according to a press release posted on the San Diego Gas & Electric Co. website. Vice President David Geier told CNN that reconstructing the events leading to the outage could take weeks, if not months.