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.

Electricity is pulsing through Spain and Portugal again after a massive outage knocked out power in both countries on Monday. So too are questions.

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."