MADRID (AP) — The cause of Spain and Portugal’s widespread blackouts remained a mystery on Tuesday, with some isolated disruption remaining after power was largely restored to both countries.

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 almost 18 hours after a blackout that shut down much of daily life. The cause of the outage remained unclear.