Red Eléctrica, Spain’s electricity grid operator, ruled out a cyberattack, human error or unusual weather. Experts say it could take weeks to determine the cause.

A blackout has brought much of Spain and Portugal to a standstill, affecting millions of people. The Spanish power distributor calls it “exceptional and extraordinary."

The hunt has begun for the cause of a massive blackout across Spain, Portugal and parts of France that halted trains, bank machines and traffic lights, in one of Europe's biggest ever power system collapses.