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.

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.

The Madrid Open will resume play this morning after the tennis tournament was hit by the vast power outage across Spain and Portugal on Monday.