A massive blackout left tens of millions of people without electricity in Argentina, Uruguay and Paraguay on Sunday in what the Argentine president called an "unprecedented" failure in the countries' power grid.
Authorities were working frantically to restore power, and by the evening electricity had returned to 90 percent of the South American country, according to Argentine state news agency Telam. Power also had been restored to most of Uruguay's 3 million people.
As the sun rose Sunday over the darkened country, Argentine voters were forced to cast ballots by the light of cell phones in gubernatorial elections.
Public transportation was halted, shops closed and patients dependent on home medical equipment were urged to go to hospitals with generators.
"This is an unprecedented case that will be investigated thoroughly," Argentine President Mauricio Macri said on Twitter.
Argentina's power grid is generally known for being in a state of disrepair, with substations and cables that were insufficiently upgraded as power rates remained largely frozen for years.
The country's energy secretary said the blackout occurred at about 7 a.m. local time when a key Argentine interconnection system collapsed.
By mid-afternoon nearly half of Argentina's 44 million people were still in the dark.
The Argentine energy company Edesur said on Twitter that the failure originated at an electricity transmission point between the power stations at the country's Yacyretá dam and Salto Grande in the country's northeast.
But why it occurred was still unknown.
An Argentine independent energy expert said that systemic operational and design errors played a role in the power 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."
Energy Secretary Gustavo Lopetegui said workers were working to restore electricity nationwide by the end of the day.
"This is an extraordinary event that should have never happened," he told a news conference. "It's very serious."
Uruguay's energy company UTE said the failure in the Argentine system cut power to all of Uruguay for hours and blamed the collapse on a "flaw in the Argentine network."
"This is the biggest blackout in history, I don't remember anything like this in Uruguay," said Valentina Giménez, a resident of the capital, Montevideo.
She said her biggest concern was that electricity be restored in time to watch the national team play in the Copa America football tournament Sunday evening.