Power was restored Friday afternoon for most of the 5 million people who were left in the dark across Southern California, Arizona and Mexico after a major transmission line failed Thursday afternoon. Officials still warned that the electrical grid was fragile and encouraged residents and businesses to go easy on major appliances. The massive blackouts extended from Orange County to San Diego County and most of the northern part of Baja California, MSNBC reported. The power company said a transmitter line between Arizona and California was severed and extreme heat in some areas also played a role in the mysterious outage. FBI officials told NBC San Diego the outage was not related to a terror threat surrounding the 10-year anniversary of the Sept. 11 attacks.