Electricity is pulsing through Spain and Portugal again after a massive outage knocked out power in both countries on Monday. So too are questions.

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.

Spain and Portugal are still reeling from the largest power cut in recent European history, which struck just after midday on Monday. With power supplies mostly back to normal, attention is laser-focused on what caused the incident.