Argentina announced an inquiry into the causes of a massive blackout that left nearly 50 million people without power as Energy Minister Gustavo Lopetegui admitted Monday it may take two weeks for the full picture to emerge.

President Mauricio Macri's center-right government still does not know what caused the problem, more than 24 hours after the outage left nearly all of Argentina and Uruguay and, briefly, parts of Paraguay in darkness.

Power to most areas was restored by late Sunday but Macri, who has targeted utilities like electricity for price hikes as part of his market friendly reforms, came in for heavy criticism on Monday.

Lopetegui meanwhile told Argentine radio that technical data from the "black boxes" of two companies responsible for electricity supply would be examined in depth, but the results of their investigation would not be known for about 15 days.

"Thousands of pieces of data" from the two companies, Cammesa and Transener, would have to be analyzed, he said.

"The two companies have to provide the report of what happened over the next 72 hours, with that information Cammesa will provide a report of what happened until 7:07 am, and in 15 days we will know what was the sequence of events that caused the blackout," Lopetegui told Radio la Red, referring to the time when the massive power failure occurred.

Investigations have so far focused on the electrical supply system from Yacyreta, a hydroelectric dam on the border with Paraguay. 

The blackout "was something serious that should not have happened," the minister said, adding that a key question was why normal safeguards did not kick in. "We have to know why the system did not act as it does every day, because failures happen every day." 

Some 44 million people in Argentina and 3.5 million in neighboring Uruguay, with which it shares a common power grid, were left without power for more than 10 hours on Sunday, before it was gradually restored.