BUENOS AIRES, Argentina (AP) — The huge blackout that left tens of millions of people in the dark in Argentina, Uruguay, and Paraguay over the weekend raised serious questions about the vulnerability of the power grid in South America and brought criticism down on Argentina’s leader.

President Mauricio Macri promised a thorough investigation into the cause of what he called an unprecedented outage. Energy officials said the findings would not be available for 10 to 15 days, and they had no immediate estimate of the economic damage from Sunday’s 14-hour power failure.

“This is an extraordinary event that should have never happened,” said Argentine Energy Secretary Gustavo Lopetegui. “It’s very serious. We can’t leave the whole country all of a sudden without electricity.”

He vowed: “There is zero chance that this will repeat itself. It can’t repeat itself.”

While the precise cause has yet to be established, the blackout originated at a transmission point between two power stations in the country’s north-east “when the system was getting too much power”, Lopetegui said. A chain of events then caused a total disruption, he stated.

Lopetegui did not rule out the possibility of a cyberattack but said it was unlikely.

Argentine energy experts said that operational and design errors probably played a role and that the system should have isolated the local failure before it cascaded so disastrously.

“If the automatic system would have been working correctly, we wouldn’t be talking about this right now,” said Daniel Montamat, a former energy secretary.

Juan Balda, a native of Argentina who is head of the electric engineering department at the University of Arkansas, speculated that a short circuit in a transmission line — caused perhaps by a fallen tree limb or lightning — set off a “domino effect”, tripping a series of protective circuits that shut down power plants one after another. A similar chain of events led to a blackout in the US North-east and Canada in 2003.

The power failure in South America happened with winter about to begin in the Southern Hemisphere. Blackouts are much more common in the summer, when the use of air conditioners pushes the grid to the maximum.

While energy officials defended the Argentine power system as “robust”, the grid had been known to be in a state of disrepair, with substations and cables that were insufficiently upgraded as power rates remained largely frozen for years.

Macri came to power in 2015 promising to reinvest in the grid and end what he called the failed energy policy of his predecessors, and outages have become less common under his government. Just over a week ago, he boasted on Twitter about Argentina’s exports of natural gas, saying: “From darkness to exporting energy.”

But after Sunday’s embarrassing outage, his rivals jumped at the chance to criticise the conservative leader, who is up for re-election in October.