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.

Electricity was back in most of the two countries after a blackout that shut down much of daily life. The cause of the outage remained unclear.