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.

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

A massive and unprecedented power outage on Monday hit large parts of Spain as well as Portugal. The blackout halted subway and railway trains, cut phone service and shut down traffic lights and ATMs for the millions of people who live across Europe's Iberian Peninsula.