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A Brazilian official says the blackout that left millions of people in the dark in Brazil and neighboring Paraguay Tuesday was caused by a lightning storm.

The Energy Ministry's executive secretary, Marcio Zimmermann, told reporters Wednesday that the storm took out three transmission lines running from the Itaipu hydroelectric dam on the Brazil-Paraguay border.

Much of the southern half of Brazil, including the two largest cities, Sao Paulo and Rio de Janeiro, was without power Tuesday night.

All of Paraguay was blacked out for a brief period.

Officials say the Itaipu plant was shut down completely for several hours.  The malfunction caused a loss of about 17,000 megawatts to the national electricity grid.

The blackout knocked out traffic signals and subway service, snarling traffic and forced evening commuters to abandon train cars.  There were also reports of a spike in robberies on city streets.

But the director of the Brazilian Center for Infrastructure Studies, Adriano Pires, says the problem is that Brazil has failed to maintain its power lines, saying a storm alone should not cause such a massive power outage.

The Itaipu hydroelectric plant - one of the largest in the world - returned to normal operation by early Wednesday.

Some information for this report was provided by AFP, AP and Reuters.