MADRID (AP) — The cause of Spain and Portugal’s widespread blackouts remained a mystery on Tuesday, with some isolated disruption remaining after power was largely restored to both countries.

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.

A massive power outage appears to have affected large portions of Spain and Portugal Monday. Red Electrica, the operator of Spain's power grid, said on social media Monday morning that voltage has been recovered in substations in several areas in the north,