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. One of Europe’s most severe blackouts grounded flights, paralyzed train systems, disrupted mobile communications and shut down ATMs across the Iberian Peninsula on Monday.

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

Red Eléctrica, Spain’s electricity grid operator, ruled out a cyberattack, human error or unusual weather. Experts say it could take weeks to determine the cause.

Play at the Madrid Open tennis tournament has been suspended because of a major power outage reported in Spain and Portugal.

MADRID (Reuters) -Play at the Madrid Open was cancelled on Monday after a widespread power outage struck the Spanish capital, forcing 15th seed Grigor Dimitrov and his British opponent Jacob Fearnley off court as scoreboards went dark and overhead cameras lost power.

Power was restored at the Caja Magica tennis complex and the Madrid Open resumed with a packed schedule that included second-ranked Iga Swiatek advancing to the quarterfinals.