(Bloomberg) -- Argentina said it isn’t ruling out a cyberattack after what President Mauricio Macri called an “unprecedented” power blackout struck five South American countries on Sunday.

Macri said Argentina is investigating the incident, which began with an as yet unexplained fault in its power grid that led to outages in Argentina, Uruguay, Brazil, Chile and Paraguay.

Though a cyberattack isn’t the primary hypothesis, it can’t be ruled out, Argentine Energy Secretary Gustavo Lopetegui told reporters in Buenos Aires. A “technical issue” or simple humidity could have triggered the breakdown, said Carlos Garcia Pereira, head of Transener, Argentina’s largest power-transmission operator.

“This morning, a fault in the coastal transmission system caused a power outage in the entire country, whose cause we cannot yet determine precisely,” Macri said in a series of comments on Twitter. “This is an unprecedented case that will be thoroughly investigated.”

Paraguay’s National Electricity Administration said the breakdown began with an as yet unexplained event in Argentina’s power network, which led to a fault that disconnected all generators at the Yacyreta hydroelectric dam from the regional grid on Sunday morning.

Underscoring the vulnerability of cross-border grids to local glitches, the outage disrupted regional elections in Argentina and cut off power for millions in South America’s second-biggest economy. The integrated nature of the regional grid meant that the interruption also affected Brazil, Chile and Paraguay, Uruguayan state utility UTE said in a statement.

That chain of events was “abnormal,” Lopetegui said.

Governor Races

Electricity was restored in about half of the affected areas in Argentina by 2:30 p.m., more than seven hours later, and is now 100% restored, according to the Energy Ministry. Power has been restored to 99% of clients in Uruguay, according to state-owned utility UTE.

The blackout delayed voting Sunday in gubernatorial elections in Argentina’s provinces of Santa Fe, Formosa, and San Luis. Voters marked ballots by candlelight or using their mobile phones as flashlights amid an “emergency protocol,” newspaper Clarin reported.

Gubernatorial elections in Tierra del Fuego in the south were unaffected by the outage because the province is separate from the rest of the country’s grid.

Businesses that were normally open on Sundays stayed shut. Phone service also was intermittent. Several panoramic photos shared on social media showed huge swaths of Buenos Aires with no lights on.

“When I started my work we didn’t have any electricity in the building, the streets were empty, not a single business was open,” Buenos Aires resident Maria Carrera said by phone around noon. “I haven’t been able to cook breakfast, and we still don’t have any power.”

The outage was set off by a fault affecting Yacyreta’s transmission system, causing power to be cut automatically and triggering a grid failure at 7:07 a.m. in Argentina, the Energy Secretariat said. That incident led to generators at a hydroelectric plant on the border of both Paraguay and Argentina suddenly going off-line.

(A previous version of this story was corrected with the accurate countries.)

--With assistance from Fabiola Moura, Ken Parks, Patrick Gillespie and Cormac Mullen.

To contact the reporters on this story: Amy Stillman in Mexico City at astillman7@bloomberg.net;Jorgelina do Rosario in Buenos Aires at jdorosario@bloomberg.net

To contact the editors responsible for this story: James Ludden at jludden@bloomberg.net, Tony Czuczka, Matthew G. Miller

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