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.

While it may be tempting to blame the unprecedented power outage that hit the Iberian peninsula this week on the rapid growth of wind and solar power in Spain, reliance on renewables is not to blame.

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."