Brazil's president says last week's massive blackouts in Latin America's largest nation were caused by a short-circuit in a transmission tower. But President Luiz Inacio Lula da Silva says it's unclear how the short-circuit happened.

The outages left nearly a third of Brazil's 190 million citizens in the dark — raising concerns about energy security for football's 2014 World Cup in Brazil, and the 2016 Olympic games in Rio de Janeiro.

Silva said Monday on his weekly radio program that the short-circuit happened in the rural Sao Paulo state town of Itabera. He says an investigation will determine why but offered no prediction on when it will be concluded.

Silva also says he will work hard to make sure similar blackouts don't happen again.