Overconfidence in the capacity of a single transmission line may have played a role in a massive power outage that left Argentina and parts of Uruguay and Paraguay in the dark three days ago, experts said Wednesday.

Argentina's government said the origin of the blackout was in northeast Argentina and hope to provide a detailed explanation of what caused the outage in about 12 days.

What is known is that just after 7 a.m. Sunday, a first failure occurred on a stretch of transmission line known as Colonia Elia Y Mercedes, which was successfully isolated. Seconds later the anomaly repeated itself in the Colonia Elia-Belgrano stretch in the city of Campana. This was not isolated and ended up shutting down the entire grid and leaving tens of millions of people across three countries without power.

The line ran parallel to and replaced one that had been out of service since April while a high-voltage transmission tower was being repaired. A bypass was installed to keep the system functioning while the repairs were carried out. Electricity from the Yacyretá and Salto Grande hydroelectric plants flowed through the working line.

"It seems to me that perhaps it was a bit imprudent to put so much energy from those two plants on a single line with no back-up or the possibility that if it failed you could count on something else," former Energy Secretary Emilio Apud told The Associated Press.

Apud said investigators should look into whether the bypass was well constructed and the failure of the protection systems that put out of service all the plants that had been working.

Raúl Bertero, president of Argentina's Center for the Study of Energy Regulation, said the operator knew the line was out of service or in "precarious" service and should have operated the system "in such a way that if something happened on the parallel line, the system was not unbalanced beyond the possibility of controlling it."

A representative of the High-Voltage Electric Power Transmission Company, who was not authorized to speak to the press and asked to not be identified, said authorities are trying to "thoroughly investigate a technical failure" and that the company will not say anything more until the investigation is completed.

The huge blackout raised serious questions about the vulnerability of the power grid in South America and brought criticism down on Argentina's leader. President Mauricio Macri has promised a thorough investigation into the outage.

Argentine energy experts said the system should have isolated the local failure before it cascaded so disastrously.

A similar outage in Brazil left more than 60 million in the dark in 2009. Three months ago, crisis-torn Venezuela suffered its worst power failure.

In 2003, about 50 million people in eight U.S. and Canada were hit by a cascading blackout that began when a tree branch in Ohio touched a power line.