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RIO DE JANEIRO (AP) -- Brazil's president suggested Friday that bad weather may not have caused a massive blackout that left nearly a third of the population without electricity, saying he'll wait for an investigation to conclude how the outage happened.

President Luiz Inacio Lula da Silva backed off earlier claims by his energy minister that strong storms, wind and lightning caused a failure in transmission lines - after the government's own satellite imagery showed that lightning strikes were neither close enough nor strong enough to cause such damage.

"There must be an investigation that follows all leads to find out what really happened," Silva told reporters in Sao Paulo. "If the system is robust and efficient as we believe, and we produce energy at will, why did we have this disaster?"

The blackout hit 18 of Brazil's 26 states and also left 7 million people without water service after energy from the massive Itaipu dam, the source of one-fifth of Brazil's power, went completely offline. In all, some 60 million people lost power in the nation of more than 190 million.

The blackouts also raised questions about whether Brazil is sufficiently prepared to provide secure energy when it hosts the World Cup in 2014 and the 2016 Olympic Games in Rio de Janeiro.

After the blackout darkened both Rio and Sao Paulo and other key cities Tuesday night, Brazilian Energy Minister Edison Lobao said it was bad weather that took out transmission lines running from Itaipu to two electric substations in Sao Paulo state. Three key transformers short-circuited, he said.

But the National Institute for Space Research said the nearest lightning strikes were six miles (10 kilometers) from any transmission line. Others pointed out that the transformers are built to withstand the heavy rains common in Brazil.

Silva said there are no solid answers yet.

"We're in a stage of searching," Silva said. "When we end this phase we will enter the more objective phase with concrete results."

Silva called for the federal investigation into the causes of the blackout on Thursday, after it became clear many people were unsatisfied with Lobao's explanation.

There were also signs of infighting within the government after the power failure.

Jorge Miguel Samek, the head of Itaipu Binacional, the agency in charge of the dam, said the problem had nothing to do with the hydroelectric project, but with a failure in the transmission lines. But, Furnas Centrais Eletricas, the electric company that oversees the lines, said it had detected no problems with the lines.

Silva called top officials to a meeting in Brasilia on Wednesday afternoon, and they all agreed that bad weather was to blame.

But by Friday, Silva was pushing hard for a thorough investigation to determine the cause.

The blackouts came three days after CBS's "60 Minutes" news program reported that two past Brazilian power outages were caused by hackers.

Lobao declined to directly answer a reporter's question about whether hackers were involved, and Silva knocked down questions about the blackout being the result of sabotage.

"There is no reason for anyone to think that it was a bigger thing than it really was," he said Friday.

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