Officials in Argentina have begun an investigation into what caused a power cut that left the entire country in the dark for a day.
The outage began at 7am on Sunday, with Argentina's population of 44 million, and many residents of neighbouring Uruguay, and some parts of Paraguay, waking up to Father's Day with no lights.
Power was restored to most of the tens of millions of people affected by Sunday night.
Gustavo Lopetegui, the Argentine energy secretary, did not rule out the possibility of a cyber attack, but said it was unlikely.
He said: "This is an extraordinary event that should have never happened. It's very serious. We can't leave the whole country all of a sudden without electricity.
"From zero to 10, there is zero chance that this will repeat itself. It can't repeat itself. We're all asking why this system didn't act as it normally does."
He said the blackout originated at an electricity transmission point between the power stations at Argentina's Yacyreta dam and Salto Grande in the country's northeast.
The system there was overloaded but a chain of events, which ha not yet been explained, then took place and caused a total disruption.
Public transportation ground to a halt in Buenos Aires, a city of 16 million people, phone and internet communications went down, water supplies were cut off, and shops were forced to close. Hospitals had to rely on generators.
The power cut raised urgent questions about flaws in South America's grid.
It came amid a spike in utility bills as Mauricio Macri, the Argentine president, cut subsidies as part of his austerity measures.
The conservative leader, who is seeking re-election in October, has seen his popularity plunge during the crisis, along with protests.
Julieta Dodda, 27, a worker at a clothes shop in downtown Buenos Aires, said: "The country is already in a weird moment and then you wake up and can't see anything."