[Updated at 9:02 p.m. ET] Authorities were investigating a power blackout in parts of southern California, Arizona and Mexico on Thursday that left more than 1 million customers without power and caused widespread traffic jams.

San Diego Gas & Electric said all of its 1.4 million customers were without power. The problem appeared to have originated in Arizona, the utility said.

"This caused our line from AZ and from the north of our region to both trip off," the utility posted on its Twitter account.

Power outages in California stretched from San Clemente to the state's border with Mexico on Thursday, a San Diego County spokesman told CNN.

In Arizona, about 56,000 customers in Yuma, lost power, Arizona Power and Supply said. Parts of Mexico's Baja California and Sonora states were also without power, Mexican authorities said.

The cause of the blackout was unclear. The San Diego utility said greater power usage during hot weather could be responsible.