What we know about the power outages in Spain and Portugal

Traffic lights went dark at busy intersections. Trains stopped in the middle of a workday. And long lines formed at ATMs and grocery stores, where credit card readers stopped working.

Widespread power outages upended life for millions across Spain and Portugal (and, briefly, parts of France) on Monday, leading to questions about what caused the disruptions and when the lights would return.

Spain’s national power company said early Tuesday that nearly all power demand in Peninsular Spain was being met as of 6 a.m. Power had begun to return to Portugal on Monday, according to that country’s electricity and gas supplier. The electrical grid operator in France, where the outage briefly affected households in the Basque region, said all power had been restored.

Here’s what we know.

The cause remains under investigation.

Spanish Prime Minister Pedro Sánchez said officials were still investigating the causes of the outage.

Portugal’s national energy supplier, E-Redes, cited an unspecified “problem in the European electricity grid.”

António Leitão Amaro, a senior official in the Portuguese government, said there was no evidence of a cyberattack. REN, a Portuguese electricity and gas supplier, denied reports that an unspecified “atmospheric phenomenon” was to blame.

Hospitals, banks and travel were disrupted in Spain and Portugal.

There were widespread problems connecting to the internet and to phone networks across Spain and Portugal, and Sánchez urged people to make only brief calls on their cellphones.

Hospitals in Spain were forced to run on generators. Portuguese banks and schools closed. Spain’s national rail company said trains had stopped operating at all stations. Subways in several cities, including Valencia and Madrid, were halted. The Madrid Open tennis tournament was suspended. And people crammed into stores to buy food and other essentials as clerks used pen and paper to record cash-only transactions.

Diana Alfia, an employee at a hostel in Lisbon, Portugal, said some people had gone to the beach since there was not much else to do, and some tourists walked miles from the airport into the city because Uber and public transportation were not available.

“I feel like everybody is still calm, because a lot of people are hoping for it to become better within the next hours,” she said. “But we don’t know in the end.”

Power went out amid high temperatures.

According to Spain’s meteorological agency, AEMET, temperatures across the country were between 60 and 70 degrees Fahrenheit around the time of the power outages. By the end of the day, highs had climbed to between 82 and 87 degrees.

This was above average: In April, temperatures in Spain average about 64 degrees in central and northern regions and about 70 degrees in the south.

Asked whether the outages were connected to the heat, Bruno Silva, a spokesperson for REN, was skeptical. “No, I hope not,” he said.

Other European outages have halted daily life.

More than 50 million people in Italy were left in the dark for almost a full day in 2003 after a line between Switzerland and Italy was overloaded. It was considered the worst day of blackouts in the country since World War II.

In 2006, 10 million people in Germany were briefly without power after the northwestern part of the country’s power grid became overloaded.

And last year, much of the Balkans was without power for several hours during a heat wave in which temperatures soared to 40 degrees Celsius (over 100 degrees Fahrenheit).

The power grids in Europe are interconnected, and an overload or issue in one area can spread to another country.