The country’s top officials are trying to figure out the cause of a power outage that stranded tens of millions of people across the Iberian Peninsula.

Spain and Portugal switched their power back on after the worst blackout in their history, though authorities offered little explanation for what had caused it or how they would prevent it happening again.

One of Europe’s most severe blackouts grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs across the Iberian Peninsula.