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 widespread power outage hit Spain and Portugal, affecting millions of people who live across Europe's Iberian Peninsula.

Power started returning to parts of the Iberian peninsula late on Monday after a huge outage brought most of Spain and Portugal to a standstill, grounding planes, halting public transport, and forcing hospitals to suspend routine operations.