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.

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.

As factories, stores and hotels across Spain and Portugal gradually returned to normal on Tuesday after a huge blackout the day before, business associations and companies began counting the cost.