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.