That conclusion has been supported by the operator of Spain’s national grid, Red Eléctrica, which has located where outages began on Monday. Two separate incidents of power outages led to instability in the grid leading to a “breakdown in interconnections with France,” leaving the Iberian grid isolated.

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.

A widespread power outage hit Spain and Portugal, affecting millions of people who live across Europe's Iberian Peninsula.