Spain's government said it had asked private energy companies for "maximum collaboration and transparency" to help identify the cause of the outage.

Some media outlets have speculated that Spain’s April 28 blackout may have been caused by a disconnected solar plant, but DNV grid analyst Andrea Mansoldo tells pv magazine that it was likely due to a combination of grid weakness and low-frequency oscillations.

The Iberian Peninsula’s widespread blackout raises questions about the resilience of the electric power infrastructure in the two countries — and to an extent, the rest of Europe.