Before dawn, power was gradually returning to several regions across Spain and Portugal as the nations reeled from the still-unexplained widespread blackout that had turned airports and train stations into campgrounds for stranded travelers.

Power was almost fully restored to Spain and Portugal on Tuesday although many questions remained about what caused one of Europe’s most severe blackouts that grounded flights, paralyzed metro systems,

The Spanish prime minister, Pedro Sanchez, said that a problem in the European grid caused the huge power outage that struck Spain, Portugal and parts of France. Mr Sanchez described the problem in the European grid as a ‘strong oscillation’ and added that the cause is still being determined.