Power had almost fully returned to Spain early Tuesday morning as many questions remained about what caused one of Europe’s most severe blackouts that grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs across Spain and Portugal.

BARCELONA, Spain (AP) — A blackout brought much of Spain and Portugal to a standstill Monday, halting subway and railway trains, cutting phone service and shutting down traffic lights and ATMs for millions of people across the Iberian Peninsula.