Portuguese grid operator REN will allow utilities to resume imports of electricity from Spain, but the transmission capacity will be limited to around one-third of the levels seen before the huge power blackout in the two countries on April 28.

Electricity is pulsing through Spain and Portugal again after a massive outage knocked out power in both countries on Monday. So too are questions.

The countries’ top officials were trying to determine the cause of an outage that left tens of millions of people across the Iberian Peninsula without power.