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.

One of Europe’s most severe blackouts grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs across the Iberian Peninsula.