In the dark, people vote in gubernatorial elections in Rosario, Argentina, on Sunday after a national power outage. Photo by Franco Trovato Fuoco/EPA-EFE

June 17 (UPI) -- Investigators can't say for sure yet what caused a power blackout for tens of millions of people in South America over the weekend, but they know it didn't come from a cyberattack.

The blackout cut electricity to more than 40 million people in Argentina, Paraguay and Uruguay on Sunday. Officials called the power failure "unprecedented."


Half of Argentina's power had been restored by late Sunday, including Buenos Aires. Lights are also being turned back on in parts of Uruguay and Paraguay. At the height of the blackout, an estimated 44 million people were without power.

"At this moment, we do not rule out any possibilities but ... a cyberattack is not within the preliminary alternatives being considered," Argentina energy minister Gustavo Lopetegui said Sunday.

Lopetegui said the blackout "shouldn't have happened" because the "Argentine system is pretty robust."

Traffic lights failed, water supplies were disrupted and mobile phones and Internet services were interrupted by the blackout, which also disrupted train and subway services. The emergency also coincided with provincial elections in some Argentinian provinces, leaving some voters to cast their ballot in the dark.

The blackout is the first in Argentina's history, Alejandra Martinez told CNN affiliate TN. It started with a collapse in the interconnection system at the Yacyreta Dam, utility Edenor said. The outage quickly spread to Paraguay and Uruguay. Some parts of Chile and Brazil were also affected.