The lights were back on Monday across Argentina, Uruguay and Paraguay after a huge blackout that affected tens of millions people, but authorities remained in the dark about the cause of the grid collapse and continued to calculate the economic damage.
Argentine President Mauricio Macri promised a thorough investigation into what he called an "unprecedented" outage, one that raised questions about flaws in South America's power grid, which connects many of the region's largest countries.
Energy officials said the results of the investigation would be available in 10 to 15 days, and they could not immediately provide details on the economic impact of the outage, which happened on a Sunday, a day before a national holiday in Argentina.
"From zero to 10, there is zero chance that this will repeat itself. It can't repeat itself," Argentine Energy Secretary Gustavo Lopetegui said Monday.
The blackout originated at an electricity transmission point between the power stations at Argentina's Yacyretá dam and Salto Grande in the country's northeast "when the system was getting too much power" Lopetegui said. Although these types of failures have happened in other countries as well, he said a chain of events took place that caused a total disruption.
"This is an extraordinary event that should have never happened," he said. "It's very serious. We can't leave the whole country all of a sudden without electricity." He did not discount the possibility of a cyberattack but said it was unlikely.
The collapse began at about 7 a.m. Sunday, with Argentina's population of 44 million and residents of neighboring Uruguay and some parts of Paraguay waking up to Father's Day in the dark.
Public transportation halted in Buenos Aires, while phone and internet communications were disrupted, water supplies were cut off and shops were forced to close. Patients dependent on home medical equipment were urged to go to hospitals with generators.
Power was restored by Sunday night. But the outage ignited questions about the vulnerability of Argentina's power system at a time when the country is going through a deep economic crisis with soaring inflation, a tumbling of the currency and a spike in utility bills fueled by Macri's decision to cut subsidies as part of his austerity measures.
The conservative leader has seen his popularity ratings plunge during the crisis. Argentines have demonstrated against the rise in utility bills, and the blackout could trigger more such protests against Macri's government just as he seeks re-election in October.
"The country is already in a weird moment and then you wake up and can't see anything," said Julieta Dodda, 27, a saleswoman at a clothing store in downtown Buenos Aires. "Many people were going to meet for lunch to celebrate the day. I saw many online who joked: 'Happy Father's Day from Edesur and Edenor, which are our electricity companies.'"
Energy officials defended the Argentine power system as "robust." But the grid had been known for being in a state of disrepair, with substations and cables that were insufficiently upgraded as power rates remained largely frozen for years.
Outages were once more common, especially during Argentina warm summer months, but had been reduced after Macri came to power in 2015 promising to reinvest in the grid and end what he called the failed energy policy of his predecessors.
After Sunday's embarrassing outage, his rivals jumped at the chance to criticize the conservative leader.
"Millions of people who have had to pay astronomical rates for electricity to benefit those in power are still waiting for the energy to return to their homes," Alberto Fernandez, a center-left presidential candidate who is seen as Macri's top rival, said on Twitter. "Just days ago, they were boasting about 'exporting energy.'" He added a recent tweet from Macri in which the president said: "From darkness to exporting energy."
An Argentine independent energy expert said that systemic operational and design errors played a role in the grid's collapse.
"A localized failure like the one that occurred should be isolated by the same system," said Raúl Bertero, president of the Center for the Study of Energy Regulatory Activity in Argentina. "The problem is known, and technology and studies (exist) to avoid it."
Although Brazil was spared this time, a similar outage in the region's largest country left more than 60 million in the dark in 2009, just as authorities scrambled to boost confidence in its infrastructure before soccer's 2014 World Cup and the 2016 Olympics.
Three months ago, crisis-torn Venezuela suffered its worst power outage, with the lack of electricity endangering hospital patients.
Other parts of the world have also been hit by major outages. Bertero said that about 50 million people were affected by a blackout in the U.S. and some provinces in Canada in 2003, and about as many were hit by another in Italy that same year.
Argentina has had a history of blackouts, but none like Sunday's failure. Only the southern archipelago of Tierra del Fuego was unaffected because it is not connected to the main grid.
Uruguay's energy company UTE said all of Uruguay lost power for hours, and it blamed a "flaw in the Argentine network."
In Paraguay, power in rural communities in the south, near the border with Argentina and Uruguay, was also cut. Authorities said service was restored by afternoon by redirecting energy from the Itaipu hydroelectric plant the country shares with neighboring Brazil.
Many residents of Argentina and Uruguay took to social media to post pictures of their cities in the dark. Others blamed the electricity companies or the government or simply lamented that their plans had been disrupted.
Several Argentine provinces had elections for governor on Sunday, which proceeded with voters using their phone screens and built-in flashlights to illuminate their ballots.
"I don't remember anything like this in Uruguay," said Valentina Giménez, a resident of the capital, Montevideo. "What is really striking about this is that no one understands well what really happened."
Associated Press writers Almudena Calatrava, Patricia Luna and Natalie Schachar contributed to this report.