Answers demanded over Brazil blackout

State prosecutors in Brazil on Thursday demanded officials explain how a huge blackout across half the country occurred, as doubts were cast on the government's argument a storm was to blame.

The investigating prosecutors belonging to the federal public ministry gave the energy ministry, the National Electrical Energy Agency and the National Grid Operator to the end of the weekend to come up with a detailed answer.

Up to 70 million people in the southern half of Brazil suddenly found themselves without power in the blackout late Tuesday.

The outage left major cities including Sao Paulo and Rio de Janeiro illuminated only by car headlights for more than three hours -- and raised questions over Brazil's preparedness to host the 2016 Olympic Games.

President Luiz Inacio Lula da Silva's government tried to draw a line under the incident by saying a strong storm over Brazil's biggest power plant, the Itaipu hydro-electric facility on the border with Paraguay, caused the problem.

"What happened was the result of atmospheric discharges, very strong rain and wind," Energy Minister Edison Lobao said Wednesday.

He claimed lightning must have shorted out three high-tension power lines feeding Brazil from Itaipu, leading to a domino effect that saw 40 percent of the country's energy disappear from the grid.

But Brazil's state satellite monitoring agency, the National Space Research Institute (INPE), and electricity companies in southern Brazil and in Paraguay challenged that theory.

Lightning from Tuesday's storm was too far from Itaipu's lines and of such a low charge that they "would not be capable of producing a line disconnection," INPE said in a statement.

INPE experts said they believed a "malfunction" was at the origin of the emergency.

Brazil's Furnas on Wednesday said no damage had been found to any high-tension lines, and called talk of storm disruption "speculation."

A spokesman for the ANDES electricity company in Paraguay -- which briefly suffered a blackout across its entire territory -- told AFP the storm argument was a "pretext" and Itaipu had been functioning normally.

Lula himself said: "There was no lack of energy production. Energy continues to be produced. We had a problem in the transmission line."