Electricity has returned to most of Spain and Portugal but with continued disruptions on the second day after an unknown failure caused a massive blackout across much of western Europe.

Spain's High Court on Tuesday said it will open an investigation into Monday's huge power outage to find out if a cyberattack against Spanish critical infrastructures may have caused the blackout in nearly all of the Iberian Peninsula.

Red Eléctrica, Spain’s electricity grid operator, ruled out a cyberattack, human error or unusual weather. Experts say it could take weeks to determine the cause.