The country’s top officials were trying to determine the cause of an outage that left tens of millions of people across the Iberian Peninsula without power.

This week’s blackout in Spain and Portugal confronted authorities with an unprecedented event: the first mass electrical outage on a grid largely powered by wind and solar energy.

Spanish Prime Minister Pedro Sanchez warned of a “long night ahead,” as the worst blackout to hit Europe in over a decade approached the 12-hour mark with no cause clearly identified and an uncertain timeline for the full restoration of power.