BUENOS AIRES — The lights were back on across virtually all of Argentina and neighbors Paraguay and Uruguay on Monday morning, according to Argentine energy officials, though the cause of Sunday's massive blackout remains a mystery.

Argentina's grid collapsed early on Sunday morning, cutting power to tens of millions of people in South America, hobbling phone and internet communication regionwide and shutting down YPF SA's La Plata refinery, Argentina's largest.

A source with Argentina's Energy Secretariat told Reuters that a failure in high tension transmission lines had "destabilized" the Yacyretá y Salto Grande power plants had caused the outage.

But the precise catalyst for the chain reaction that eventually blacked out much of the southern half of South American is still unclear, according to Energy Secretary Gustavo Lopetegui.

"We're all asking why this system didn't act as it normally does, because failures like this happen every day," Lopetegui told local radio station La Red.

Lopetegui said that the results of an investigation into the causes of the blackout were due in 15 days, but said it was impossible that such a widespread outage could occur again.

Residents of Buenos Aires, a city of nearly 16 million, awoke on Father´s Day to a nearly entirely dark city. Power trickled back over several hours there and across much of Argentina, and was fully restored by around 9:30 p.m. local time, according to the Energy Secretariat. — Reuters