Spain’s grid operator offered the most detailed account yet of the events that led to a blackout that spilled over to Portugal, describing a series of disruptions over just a few seconds that plunged the country into darkness for hours.

The information still leaves major questions about the root of the faults unanswered, but it shows how stress on the network — which has since been restored — makes it vulnerable and will need investment to become more resilient.

“These are not definitive conclusions, we don’t have the complete information,” Eduardo Prieto, director of services and operations at Spanish grid operator REE, said Tuesday before describing the events that preceded the blackout.

Adding to the confusion was conflicting views over a possible cyberattack. While the grid operator ruled out a hack, public officials were more cautious. A judge in Spain’s national court opened an investigation, and Prime Minister Pedro Sanchez pleaded for patience.

“It would not be wise to say it was a cyberattack nor to rule it out,” Sanchez said on Tuesday. “The important thing is to have exact knowledge of what happened in those five seconds and from there make the necessary decisions.”

The search for what exactly caused the faults will be critical to determine how to better protect the system. That’s key not just for Spain, but for all of Europe, where an increasingly interconnected power network allows countries to help each other during times of stress, but also raises the risk that problems in one nation flow into another.

A cascade of failures started at just after 12:33 p.m. in Madrid on Monday. The initial event was consistent with a loss of power generation, which occured in southwest Spain, according to grid operator REE, which didn’t provide details over the exact site or the type of generation.

While the system recovered as it should, just 1.5 seconds later another event occurred — likely a loss of more generation. Then 3.5 seconds later, there was a disruption in the connection between Spain and France, which isolated the peninsula from Europe’s power grid.

With Spain and Portugal isolated, there was a big disconnection of renewable power generation, triggering a severe imbalance and a system-wide collapse.

While the time line doesn’t reveal what actually caused each fault, it does provide some key details, showing how problems in the south were exacerbated by the issue with the French interconnection in the north.

That means the grid was under stress on multiple fronts, and while it was able to deal with the first disruption, the quick succession of incidents left the network without enough tools to keep the lights on, plunging millions into darkness and disrupting traffic, airports and phone service across two countries for hours.

Power was restored in both Spain and Portugal by the morning and public services returned to normal, but the fallout is still rippling across the country.

“The crisis is not over,” Sanchez said. “Our goal is, firstly, to continue the investigation to find out the causes, the origins of this electricity crisis, and secondly, to consolidate the normality toward which we are already firmly moving.”

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—With assistance from Rodrigo Orihuela, Laura Millan and Daniel Basteiro.

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