Second update: Power restored.

Update: Up to six million people could be without power

After a widespread blackout hit southern California, Arizona and parts of Mexico on Thursday night, San Diego Gas & Electric urged subscribers to "activate" their family emergency plans, warning that power could be off until tomorrow.

Update: SDG&E says that some customers will have service restored by later tonight, most by Friday afternoon. Watch live developments here.

"Essentially we have two connections from the rest of the world: One of from the north and one is to the east," a spokesman for San Diego Gas & Electric (SDG&E) said. "Both connections are severed."

The outage left over 1.4 million without power, snarling traffic and causing flights to be canceled. San Diego City Hall was also voluntarily evacuated.

Authorities cautioned that the outage was not related  to an active terrorist threat ahead of the 10th anniversary of the 9/11 attacks. They speculated that it may have something to do with the intense heat seen across the region in recent days, and techs pinpointed the outage to transfer lines in Arizona.

SDG&E said it may be tomorrow before power is restored. Meanwhile, San Diego officials are asking residents to stay off the roads, avoid using telephone land lines and cell phones and turn off their air conditioners and other electric devices to prevent overload when the power is restored.

Update: Forbes reported Friday that there were up to six million people without power.

Power officials said that a single employee in Arizona may have caused the outage by removing monitoring equipment from a power substation.

"This was not a deliberate act. The employee was just switching out a piece of equipment that was problematic," Arizona Public Service vice president Daniel Froetscher said.

Second update: SDG&E says that power has been restored.

Image credit: Flickr commons.