Electricity is pulsing through Spain and Portugal again after a massive outage knocked out power in both countries on Monday. So too are questions.

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

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.