MADRID, April 29, (AP): Power had almost fully returned to Spain early Tuesday morning as many questions remained about what caused one of Europe’s most severe blackouts that grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs across Spain and Portugal. By 7:00 a.m. local time, more than 99% of energy demand in Spain had been restored, the country's electricity operator Red Eléctrica said.

Portuguese grid operator REN said Tuesday morning all of the 89 power substations were back online since late last night after an "absolutely unusual” blackout and power was restored to all 6.4 million customers. On Tuesday morning, as life slowly returned to normal on the Iberian Peninsula, outside Atocha station, one of the busiest in the Spanish capital, people were once again crossing the street while looking at their cell phones, despite the internet connection being intermittent at times.

Inside, hundreds of people waited to board trains or rebook those that had been canceled the previous day. Large groups of people milled around the screens, waiting for updates. Still, several people slept on the floor with blankets provided by the Red Cross for those who waited overnight at the station. The same scenes were replicated at Sants station in Barcelona.

Classes were suspended in several regions. The Madrid Open, the tennis tournament being held this week, was still affected by the power outage after its cancellation the previous day and delayed the opening of its doors. Power had gradually returned to several regions across Spain and Portugal as the nations reeled from the still-unexplained widespread blackout that had turned airports and train stations into campgrounds for stranded travelers.

Monday night, many city residents, including in Spain’s capital of Madrid, went to sleep in total darkness. The normally illuminated cathedral spires of Barcelona’s Sagrada Familia Basilica became indistinguishable from the night sky. Streets remained deserted even in neighborhoods where lights flickered back on, as people stayed home after a day of chaos.

"We have a long night ahead,” Spanish Prime Minister Pedro Sánchez said when he addressed the European nation late Monday. "We are working with the goal of having power restored to the entire country.” In Madrid, cheers erupted from balconies where the electricity had returned. On Tuesday morning, Madrid's metro system was restored on all but one line, meaning that 80% of trains operated during rush hour.