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.

Alanna Gladstone, a 40-year-old film editor, had landed in Portugal’s capital, Lisbon, on a flight from New York hours before the outage. She checked into her Airbnb and took a nap; by the time she woke up, the technology that the country takes for granted had gone dark.

Spanish Prime Minister Pedro Sánchez says “no hypothesis is being ruled out” as he vowed to investigate and uncover the cause behind the massive blackout that gripped the entire Iberian Peninsula on Monday.