More than 1 million people in southern California are getting ready to spend the night in the dark after a major power failure knocked out electricity in and around the city of San Diego.

Officials blame a transmission line in Arizona which automatically shut down two reactors at the San Onofre nuclear power plant near San Diego. They are ruling out terrorism, but say they do not know what caused the line to fail. The power company says it is working to get the lights back on and is telling residents to expect to spend the night without electricity.

The blackout cut power to much of San Diego, parts of Arizona, and south into Baja California and Tijuana in northern Mexico.

The power went out just before Thursday afternoon's rush hour, snarling downtown traffic and shutting down streetcar service. Radio and television stations were knocked off the air and flights at local airports grounded.