Updated at 5:00 p.m. ET:
Argentina's energy secretary Gustavo Lopetegui says more than half the country's power has been restored after an unprecedented power failure plunged tens of millions of people into darkness Sunday. He said he does not know what happened.
"We don't believe it was a cyber attack," Lopetegui said. He added that the outage "shouldn't have happened," because "the Argentine system is pretty robust."
NPR's Philip Reeves reports that electricity has been restored in parts of southern and northwestern Argentina, but getting the lights on in the entire country will likely take hours.
Uruguay, which shares Argentina's electrical grid, also faced widespread blackouts.
Authorities say a power line at a regional hydroelectric dam went down.
At about 7 a.m., the Edesur electric utility tweeted that "a massive failure" left Argentina and Uruguay without power. The BBC reported that parts of Paraguay were also affected.
In Buenos Aires, the Constitucion railway station was empty, with trains halted, Bloomberg reported.
Traffic lights failed across the city, and some shopkeepers ran generators to keep the lights on. Water supplies were disrupted, as were mobile phone and internet services.
The Argentinian news site Infobae reported that the power cut stopped trains and subway service; however, two airports in Buenos Aires continued to run on generators.
Uruguay's state energy department wrote that "a flaw in the Argentine network" left Uruguay without light, according to Infobae.
The power outage fell on a day of provincial elections in some of Argentina's provinces.
The cause of the failure is not clear.
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