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.

Electricity was back in most of the two countries almost 18 hours after a blackout that shut down much of daily life. The cause of the outage remained unclear.

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.