Spain and Portugal lost power shortly after 12:30 p.m. local time in a massive blackout that idled public transport, airports and phone services, bringing the daily activities of millions to a halt. While some areas across Spain gradually came back online in the afternoon, restoring all supply in the country was to take until at least the evening, according to grid operator Red Electrica.

The cause of the outage is still under investigation. While officials suspect that sharp swings in power were responsible for the worst blackout in Europe in years, a cyber attack hasn’t been entirely ruled out.