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.

Spain and Portugal switched their power back on after the worst blackout in their history, though authorities offered little explanation for what had caused it or how they would prevent it happening again.