Brazil's energy minister says a blackout that left 60 million people in the dark was caused by bad weather.

Edison Lobao says heavy rains and strong winds caused three transformers on a key high-voltage transmission line to short circuit, cutting the line.

Two other lines went down as a part of an automatic safety precaution. That eventually caused the massive Itaipu hydroelectric dam — to which the lines were connected — to shutdown as well.

Lobao said Wednesday that 40 percent of Brazil's total energy was cut, in some areas for up to four hours.

The blackout that began late Tuesday has prompted questions about the robustness of Brazil's energy grid, just weeks after the nation won the right to host the 2016 Olympic Games.