More than 2 million people on both sides of the U.S.-Mexico border were left without power Thursday afternoon after a major outage that extended from Arizona to southern California, including San Diego. The outage started shortly before 4 p.m. PDT.

San Diego bore the brunt of the blackout and most of the nation's eighth-largest city was darkened. All outgoing flights from San Diego's Lindbergh Field were grounded and police stations were using generators to accept emergency calls across the area, according to an AP report.

As of tonight, San Diego Gas & Electric does not know what caused the outage. At about 9:15 p.m., SDG&E stated on Twitter that all power to its customers was out, but restoration of the transmission system is underway.

Mike Niggli, chief operating officer of San Diego Gas & Electric Co. ruled out terrorism but said the cause is unclear. "To my knowledge this is the first time we've lost an entire system," he said at a news conference.

The power outage caused the San Onofre nuclear-power plant operated by Edison International's Southern California Edison utility to automatically shut down at about 3:38 p.m. PDT, SoCal Edison spokesman Paul Klein said. He said the power plant's two units had enough off-site power to operate the plant's safety systems, but he declined to provide details.

The California Independent System Operator Corporation (ISO) issued a transmission emergency after a major system disturbance occurred between Arizona and Southern California and caused all of San Diego Gas and Electric (SDG&E) and a small portion of Southern California Edison customers to lose power.

The California ISO stated that the outage was triggered after a 500 kV high-voltage line from Arizona to California tripped out of service. The transmission outage cut the flow of imported power into the most southern portion of California resulting in widespread outages in the region.

The ISO said it was coordinating with SDG&E as well as neighboring utilities including Southern California and utilities in Arizona and Mexico, during restoration of service, which is being managed by SDG&E.