Power has been restored to tens of millions of people across almost all of Spain and Portugal, but disruptions to transport services were expected to stretch into a second day as questions mount over what caused a massive blackout.

It may take a week for the network servicing both countries to be fully repaired. Meanwhile, millions are without power.

Portugal’s grid operator Redes Energéticas Nacionais (REN) said electrical supply was lost across the entire Iberian Peninsula, and in parts of France, shortly after midday. Hours later, Spain’s Prime Minister Pedro Sanchez said authorities were still not sure what caused the blackout.