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Millions of people in Southern California, Arizona and Mexico were without power Thursday.

The outage started around 4 p.m. Pacific time and now we know that a single worker's error led to a massive power outage that left millions of people in the dark.

ABC News reports that the North Gila-Hassayampa 500 kV transmission line near Yuma, Ariz. was tripped offline when a single APS employee was carrying out a procedure in the North Gila substation, according to Arizona Power Service.

A spokesman for San Diego Gas and Electric company originally said that crews thought the outage was caused by a system breakdown.

Trains were stopped on the tracks and police officers were standing in city intersections directing rush hour traffic.

All outgoing flights from San Diego's Lindbergh Field were halted and police stations were using generators to accept emergency calls.

Officials reassured residents that it was a technical problem and NOT the result of a terrorist attack.

Power in some areas may not be restored until Friday.

(ABC News contributed to this story.)