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.

Millions were plunged into darkness in Europe on Monday as a massive power outage impacted parts of Spain, France and Portugal. The blackout wreaked havoc on travel across the three countries, and the impacts extended beyond there, too.