Power has been restored to San Diego and other areas in Southern California, Arizona and northern Mexico after a nearly 12-hour outage on a sweltering summer day.

Officials say they are still investigating the cause of the blackout, which snarled rush hour traffic when it began Thursday afternoon.

A spokesman for the power company Arizona Public Service said authorities suspect human error. But he added that the systems that should have guarded against such a widespread failure are likely also to blame.

Some 5 million people are estimated to have been affected by the outage, which ended in most areas before dawn on Friday.

It forced the automatic shutdown of a nuclear plant, grounded flights, and knocked radio and television stations off the air. The failure also caused a large sewage spill that closed some beaches.

Officials are cautioning those who have regained power to be conservative about its use, saying the system is still fragile.