As lights turned back on across Argentina, Uruguay and Paraguay after a massive blackout that hit tens of millions of people, authorities were still largely in the dark about what caused the collapse of the interconnected grid.

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 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 that they could not immediately provide details on the economic impact of the outage, which came on Sunday, a day before a national holiday in Argentina.

Argentine Energy Secretary Gustavo Lopetegui said the blackout began with a failure in the country’s “interconnection system”, adding it happens in other countries as well. But he said a chain of events took place later, causing a total disruption.

“This is an extraordinary event that should have never happened,” Lopetegui told a news conference. “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.

In the dark

The collapse began at about 7am on Sunday with Argentina’s population of 44 million and residents of neighbouring Uruguay and some areas 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 fully restored by Sunday night. But the outage ignited questions about Argentina’s preparedness and lack of investment in the power system at a time when the country is going through a deep economic crisis with soaring inflation, a tumbling local currency, and a spike in utility bills fueled by austerity measures ordered by Macri.

The conservative leader has seen his popularity ratings plunge during a crisis where he has struggled to tame one of the world’s highest inflation rates and poverty has reached about one-third of the population. Argentines are also frustrated with high utility costs and the blackout could trigger more 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.

‘Problem is known’

An Argentine independent energy expert said systemic operational and design errors played a role in the power grid’s collapse.

“A localised failure like the one that occurred should be isolated by the same system,” said Raul 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.”

Uruguay’s energy company UTE said the failure in the Argentine system also cut power to all of Uruguay for hours, blaming the collapse on 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. The country’s National Energy Administration said service was restored by afternoon by redirecting energy from the Itaipu hydroelectric plant the country shares with neighbouring Brazil.