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.

The sweeping power outage that hit Spain and Portugal this week has raised questions about the electricity grid in a region not normally known for blackouts.

Spain and Portugal experienced a widespread power outage on Monday, leaving millions without electricity in a snap blackout. In Portugal, official sources told domestic media that the outage, which hit around late morning local time,