An electric grid failure described by Argentine President Mauricio Macri as “unprecedented” triggered a blackout affecting tens of millions across South America Sunday.

The blackout reportedly began at approximately 07:00 a.m. local time Sunday, halting trains and plunging millions of people into darkness. A failure in an electrical grid serving both Argentina and Uruguay caused the outage, although countries including Chile, Brazil, and Paraguay were also affected.

The blackout occurred as voters prepared to go to the polls in Argentina’s local elections, forcing them to cast ballots through the light of their cellphones. Argentina halted all public transportation, shops closed, and authorities told patients dependent on home medical equipment to go to hospitals with electrical generators.

“This morning there was a power outage across the country due to a failure in the coastal transport system, the causes of which we cannot yet specify,” President Macri wrote on Twitter. “We’re working so everyone can get energy as soon as possible.” Esta mañana se produjo un corte de energía eléctrica en todo el país debido a una falla en el sistema de transporte del litoral, cuyas causas aún no podemos precisar. Estamos trabajando para que todos puedan tener energía lo antes posible. — Mauricio Macri (@mauriciomacri) June 16, 2019 By Sunday evening, power had returned to approximately 90 percent of Argentina, as energy workers scrambled to restore the electrical grid back to normality. “This is an unprecedented case, which will be thoroughly investigated,” Macri later added. Argentina’s electrical grid is generally accepted to be in a state of disrepair, with some groups blaming the ongoing economic crisis for a delay in upgrades. One local independent energy expert told CBS that systemic operational and design errors were the main factors behind 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. “The problem is known and technology and studies (exist) to avoid it.”

Power was restored to most of Argentina after a massive blackout left the entire country without electricity on Sunday. The outage also affected Uruguay and Paraguay. Local groups blame delays in power grid upgrades as the country grapples with an economic crisis. pic.twitter.com/TGmIM6u1Az — AJ+ (@ajplus) June 17, 2019

The blackout comes amid growing concerns over the ability of foreign actors to disrupt other nation’s power supply through cyber attacks. Argentina’s Energy Minister, Gustavo Lopetegui, claimed that the country’s electrical system was “very robust,” but admitted that the exact cause of the blackout remained unclear. “At the moment we’re not ruling out any possibility,” he told reporters. “But we don’t think it is down to a cyber attack.”

One South American nation unaffected by the power cut was Venezuela, which has experienced rolling blackouts in recent months as a result of failures within the country’s electrical grid. Socialist dictator Nicolás Maduro has repeatedly blamed the blackouts on the United States but has never provided any evidence for such an assertion.

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