Millions of people in Argentina and Uruguay woke up Sunday morning without electrical power after what an Argentinian national energy supplier called “a massive failure of the grid.”

While sporadic, small-scale blackouts are not unheard of in that part of the world, the sheer scale of the outage was “unprecedented,” as Argentinian President Mauricio Macri said. The outage delayed local elections in some provinces of Argentina and disrupted daily life for millions.

It took several hours for authorities to begin restoring the power supply across the two countries. It’s still unclear exactly how the power outage happened, and why it caused such widespread disruption.

Here’s what we know so far about Argentina and Uruguay’s blackout.

How many people were affected?

44 million people live in Argentina and another 4 million in Uruguay. The outage affected all of both countries, apart from the Tierra del Fuego, a sparsely populated archipelago off the southern tip of mainland Argentina that has its own electrical system. The power supply in some towns in southern Paraguay and Brazil was also interrupted.

Buenos Aires, the Argentine capital, was particularly badly hit. Train and subway services were suspended while cars and buses crawled along unaided by signal lights. Thousands lost water access, because the city’s water system relies partly on electrical pumps. Elevators in apartment blocks stopped working, leaving elderly and less mobile people unable to move in and out of their homes.

90% of Argentina had power by Sunday night, the country’s state news agency reported, while Uruguay’s state energy utility said 98.5% of services had been restored.

Where did the power cut start and how did it take down the entire grid?

It’s not entirely clear what caused such a widespread loss of power. Argentina’s energy ministry claimed the outage had begun with “a failure in the transmission of electricity” from the Yacyretá Dam, a power plant on the Paraná River along the Argentina-Paraguay border. The binational body that runs the Yacyretá Dam denied that its power station had caused the problem, saying it was in normal working order.

Argentinian newspaper Clarín cited energy ministry sources as saying that recent coastal storms had damaged the power lines connecting Yacyretá with another hydroelectric power station, Salto Grande, around 300 miles south on the Argentina-Uruguay border. The failure in transmission triggered an “alteration in the frequency” and automatic safety responses kicked in at power stations across the country, the source added.

“These kind of faults happen regularly, both in Argentina and other countries,” Argentina’s energy secretary Gustavo Lopetegui said at a press conference Sunday. “What is unusual or extraordinary is the chain of events that happened next and led to the total disconnection.”

Lopetegui added that just milliseconds had passed from the “destabilization” of the grid to the power being cut, and that no human intervention had taken place. “These are protection systems that trigger a total switch-off. We need to know why it happened,” he said.

Early Monday, Lopetegui claimed there was “zero” chance of such a large-scale outage happening again. “This was extraordinary. It’s something very serious that shouldn’t have happened, that should not happen,” he said.

How did the fault affect two countries?

Argentina and Uruguay’s power systems are closely interlinked, allowing the neighbors to buy and sell energy from one another, balancing out surpluses and shortfalls.

“The system evaluates the relation between the generation of and demand for energy second by second. When the demand falls, the generation falls by the same degree,” Uruguay’s state-owned energy company said in an explanation posted to Twitter. “As soon as the massive fall occurred in Argentina, protections kicked in automatically in the Uruguayan system.”

The same thing happened in parts of Paraguay, Brazil and Chile, where the grid is also linked to the Argentine system.

Was it a cyberattack?

Authorities weren’t ruling out any possible causes of the failure in their investigation, but Lopetegui, the Argentine energy secretary, said a cyberattack was unlikely. “It’s not among the first set of options we’re considering,” he said.

The results of the investigation are expected to take 10-15 days, he added.

Write to Ciara Nugent at ciara.nugent@time.com.